ci: Add Valgrind jobs on macOS ARM64

[real-or-random]

• valgrind-macos has finally merged support for ARM64.
• GitHub Actions offers ARM64 jobs for macos-14 and macos-15

This seems more useful than the current macOS 13 Valgrind jobs on x86_64. Almost all our tests run on x86_64 anyway, so the only diversity the jobs add is the OS (but we barely interact with the OS), and the Apple clang compiler.

The only caveat: valgrind-macos's support has a bug that may make it eat your memory and crash your machine in some cases. Since we run on GHA, there's no risk for us. We can just try and see if it works. According to the maintainer, it's more likely to occur on macos-15.

We'll need to set a special environment variable to opt in, also during the build process. (If we want to continue using brew, we could fork or patch https://github.com/LouisBrunner/homebrew-valgrind).