use gix_path::env::shell() to get the shell in git2-hooks

[Joshix-1]

This deals with one half of #2611

It changes the following:

I followed the checklist:

• I added unittests
• I ran make check without errors
• I tested the overall application
• I added an appropriate item to the changelog

[naseschwarz]

Thanks @Joshix-1. I have just one remark.

[Byron]

LGTM! Thanks again!

Please note that gix-path will probably increment its major version every couple of months, but I don't expect any actual breakage here so that should be fine.

[extrawurst]

Thanks everyone!