fix(git): respect GIT_SSH_COMMAND and GIT_SSH

[cwstra]

What's the problem this PR addresses?
At present, berry does not respect the GIT_SSH_COMMAND or GIT_SSH environment variables, nor the .gitignore file's core.sshCommand setting. In the original issue, this prevented a user from using plink instead of ssh, and I found the issue when I tried (and failed) to override which ssh command I wished to use on windows.

(Partially) Resolves #3271.
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How did you fix it?
git's behavior around the two environment variables is as follows:

• If GIT_SSH_COMMAND is set, treat that variable as a full command (including flags) to be run in place of ssh
• Otherwise, if GIT_SSH is set, treat that variable as the path to the executable to use in place of ssh. It is not expected to include flags.

As such, the straightforward fix for berry seems to be:

• If GIT_SSH_COMMAND is set, use that in place of the existing command.
• Otherwise, if GIT_SSH is set, replace the ssh in the existing command with the value of GIT_SSH.

Checking the .gitignore file would be ideal, but more involved, and respecting the environment variables at least provides a method for the user to force their own command in place of the standard.

Checklist

• I have read the Contributing Guide.

• I have set the packages that need to be released for my changes to be effective.
• (I went with patching everything that the cli suggested, which might have been a bit aggressive.)

• I will check that all automated PR checks pass before the PR gets reviewed.