Preserve executable bit in files

Some commit hooks such as
`pre-commit-hooks/check-executables-have-shebangs` are designed to run
only on executable files via `types = ["executable"]` and the python
tool `identify` uses the executable bit of a file when generating file
tags, so `executable` is only listed when that bit is set.

By removing `--no-preserve=mode` we get to keep the executable bit. We
still need everything to be writable though.

modules/pre-commit.nix

@@ -163,7 +163,10 @@ let
     runCommand "pre-commit-run" { buildInputs = [ git ]; } ''
       set +e
       HOME=$PWD
-      cp --no-preserve=mode -R ${cfg.rootSrc} src
+      # Use `chmod +w` instead of `cp --no-preserve=mode` to be able to write and to
+      # preserve the executable bit at the same time
+      cp -R ${cfg.rootSrc} src
+      chmod -R +w src
       ln -fs ${configFile} src/.pre-commit-config.yaml
       cd src
       rm -rf src/.git