modules: pre-commit: make pkgs.buildEnv composable

Passing a git-hooks.lib.<SYSTEM>.run derivation as a path to
pkgs.buildEnv results in the following error due to derivations with an
empty file being treated differently from derivations with an empty
directory:

    The store path <TARGET> is a file and can't be merged into an
    environment using pkgs.buildEnv!

Since Nixpkgs rejected explicitly handling derivations with an empty
file in pkgs.buildEnv [1], affected derivations should transition from
'touch $out' to 'mkdir $out' to be pkgs.buildEnv composable.

This change replaces 'touch $out' with 'mkdir $out' and makes the
following command work:

    nix \
      build \
      --expr '
        let
          flake = builtins.getFlake (toString ./.);
          system = builtins.currentSystem;
        in
          flake.inputs.nixpkgs.legacyPackages.${system}.buildEnv {
            name = "";
            paths = [(flake.outputs.lib.${system}.run {src = ./.;})];
          }
      ' \
      --impure

[1]: NixOS/nixpkgs#325140

Link: #498

modules/pre-commit.nix

@@ -78,7 +78,7 @@ let
       fi
       exitcode=$?
       git --no-pager diff --color
-      touch $out
+      mkdir $out
       [ $? -eq 0 ] && exit $exitcode
     '';