fix: dfx cache list: skip cache directories being installed

[ericswanson-dfinity]

When dfx installs a cache directory, it first copies all of the files into a directory _<version>_<random>, and once all files have been copied, renames the directory to <version>. This prevents dfx from later using partially copied cache directories.

This change makes dfx cache list ignore these temporary directories, rather than exiting with an error message like unexpected character '_' while parsing major version number.