Fix failures in license scripts being lost

[martinling]

Errors returned from rust_licenses.py were being ignored, because on Windows runners, commands are executed by PowerShell, and there's no equivalent of set -e there, so a multi-line run command will ignore errors as long as the last command in the sequence succeeds. This is documented here. What a footgun!

Fix this by forcing the shell to bash (thanks @miek for the tip), and then fix the underlying failure, which was that I hadn't added BSD-2-Clause to the list of accepted licenses when adding the git-version crate in #125. Also add COPYING to the list of filenames to check for a license in the crate source, which is also needed to accept this crate.

We're about to remove the git-version dependency in #126 anyway, but these changes are valid regardless.