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
