Use $NIX_PREFETCH_GIT_CHECKOUT_HOOK to produce a `.git-version' file.

Dependencies

• [2] JTRG7RDQ add support for git as jobinput
• [*] OOQ2D3KC * Refactoring: move fetchInput out of hydra_scheduler into a separate

Change contents

• edit in src/lib/Hydra/Helper/AddBuilds.pm at line 241
  [2.1120]

  [2.1120]
  + $ENV{"NIX_PREFETCH_GIT_CHECKOUT_HOOK"} = "git describe > .git-version";