feat(git-hooks): set repo remote fallback with env variable

[6d7a]

Closes #1158

Context

In some configurations ggshield runs in repositories without a configured remote, for example when running in a git pre-receive hook.

What has been done

This commit adds a REPOSITORY_REMOTE_FALLBACK environment variable for setting a fallback value for the remote URL.

PR check list

• As much as possible, the changes include tests (unit and/or functional)
• If the changes affect the end user (new feature, behavior change, bug fix) then the PR has a changelog entry (see doc/dev/getting-started.md). If the changes do not affect the end user, then the skip-changelog label has been added to the PR.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.01%. Comparing base (7218450) to head (00d20ca).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1159      +/-   ##
==========================================
+ Coverage   91.97%   92.01%   +0.04%     
==========================================
  Files         144      144              
  Lines        6202     6209       +7     
==========================================
+ Hits         5704     5713       +9     
+ Misses        498      496       -2     

Flag	Coverage Δ
unittests	92.01% <100.00%> (+0.04%)	⬆️

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