Fix issue #17: Missing path during pre-commit

[MattiasKockum]

Previous workflow generated this command:

nix-shell -A pre-commit --run 'set -o pipefail
  set -o nounset
  set -o errexit
  pre-commit run --all-files --hook-stage pre-commit --show-diff-on-failure'

Tries to call pre-commit of pre-commit : error: attribute 'pre-commit' in selection path 'pre-commit' not found

The fix here skips nix-actions.lib.nix-shell abstraction to build the right command directly.