pre-commit: use absolute path for binary in hook

This changes the generated Git hook to refer to the `pre-commit` binary
by its absolute path. This means that Git hooks created with
`nix-shell --run 'pre-commit install'` or similar will be usable outside
of the Nix shell they were created in.

I think this is the intended behavior for this package, considering that
the `postPatch` phase already includes a substitution for this variable,
otherwise unused:

    substituteInPlace pre_commit/resources/hook-tmpl \
      --subst-var-by pre-commit $out

0 files changed, 0 insertions, 0 deletions