Binary Ninja installation path detection does not work on macOS

[xusheng6]

I ran capa on macOS with the binja backend but it does not seem to detect the installation path correctly. Here is the output:

% ./capa -d -b binja ~/Downloads/[private].bndb
DEBUG    capa: --------------------------------------------------------------------------------                                                      main.py:475
DEBUG    capa:  Using default embedded rules.                                                                                                        main.py:476
DEBUG    capa:  To provide your own rules, use the form:                                                                                             main.py:477
DEBUG    capa:                                                                                                                                       main.py:478
DEBUG    capa:      `capa.exe -r ./path/to/rules/  /path/to/mal.exe`.                                                                                main.py:479
DEBUG    capa:                                                                                                                                       main.py:480
DEBUG    capa:  You can see the current default rule set here:                                                                                       main.py:481
DEBUG    capa:                                                                                                                                       main.py:482
DEBUG    capa:      https://github.com/mandiant/capa-rules                                                                                           main.py:483
DEBUG    capa: --------------------------------------------------------------------------------                                                      main.py:484
DEBUG    capa.features.extractors.common: unknown file format: 53514c69                                                                             common.py:85
DEBUG    capa.rules: reading rules from directory /var/folders/0b/0ccr7qgn5p1_vvjrz7mn74z00000gn/T/_MEIANcNkQ/rules                             __init__.py:2167
DEBUG    capa.rules.cache: loading rule set from cache: /var/folders/0b/0ccr7qgn5p1_vvjrz7mn74z00000gn/T/_MEIANcNkQ/cache/capa-c2011a71.cache       cache.py:157
DEBUG    capa: successfully loaded 974 rules                                                                                                         main.py:693
DEBUG    capa.features.extractors.common: unsupported file format: 53514c69, will not guess OS                                                     common.py:148
DEBUG    capa: skipping library code matching: only supported by the vivisect backend                                                                main.py:817
DEBUG    capa: format:  binja_database                                                                                                               main.py:867
DEBUG    capa: backend: binja                                                                                                                        main.py:868
DEBUG    capa.features.extractors.binja.find_binja_api: Binary Ninja API not installed, searching...                                       find_binja_api.py:156
WARNING  capa.features.extractors.binja.find_binja_api: unsupported platform to find Binary Ninja: darwin                                  find_binja_api.py:114
DEBUG    capa.features.extractors.binja.find_binja_api: failed to find Binary Ninja installation                                           find_binja_api.py:160
DEBUG    capa.features.extractors.binja.find_binja_api: found Binary Ninja API: None                                                       find_binja_api.py:162
Traceback (most recent call last):
  File "main.py", line 1149, in <module>
  File "main.py", line 1033, in main
  File "main.py", line 871, in get_extractor_from_cli
  File "loader.py", line 254, in get_extractor
RuntimeError: cannot find Binary Ninja API module.
[PYI-76527:ERROR] Failed to execute script 'main' due to unhandled exception!

I checked the source code and noticed a few things:

1. is_binaryninja_installed should return True, since I can run importlib.util.find_spec("binaryninja") get the result just fine. Not sure why it does not work
2. find_binaryninja_path_via_subprocess actually obtains the correct path of the binja python modeule, i.e., in /Applications/Binary Ninja.app/Contents/Resources/python (the default installation). However, the validate_binaryninja_path method tries to append another python into the path, so it breaks it. I see that validate_binaryninja_path is also used to validate the binja path obtained from the Linux desktop file. Maybe the path obtained there does not already have the python in it so it works? Anyways, the difference here needs to be fixed
3. We should also add macOS and Windows binja path detection by looking at the default installation

[williballenthin]

I just recently got access to a macOS system, so I can test/triage this, too.