fix: allow .git directory on parent directory

Currently, the check to validate pre-commit-hooks are getting installed
on a git repository only consider the `.git` directory, if the
pre-commit-hooks config lives in another path that is not the root of
the repository, pre-commit-hooks won't install a script.

This change allows flake.nix files in sub-directories to have the
pre-commit-hooks configuration. This is specially common in monorepos.

modules/pre-commit.nix

@@ -366,7 +366,7 @@ in
           if ! type -t git >/dev/null; then
             # This happens in pure shells, including lorri
             echo 1>&2 "WARNING: pre-commit-hooks.nix: git command not found; skipping installation."
-          elif [[ ! -e .git ]]; then
+          elif ! ${git}/bin/git rev-parse --git-dir &> /dev/null; then
             echo 1>&2 "WARNING: pre-commit-hooks.nix: .git not found; skipping installation."
           else
             # These update procedures compare before they write, to avoid