develop: Made the run parameter optional

There's a valid use case for wanting to run `nix develop` without
executing anything: to get the dependencies installed. This way the
output of future steps will have considerably cleaner, more relevant
outputs.

Signed-off-by: Gergely Nagy <me@gergo.csillger.hu>

.forgejo/workflows/nix-develop.yaml

@@ -30,3 +30,9 @@ jobs:
             if [ -n "${CI}" ]; then
                 echo "I'm running in CI!"
             fi
+
+      - name: no commands should still work
+        uses: ./develop
+        with:
+          flake: ./develop/test
+          package: "hello"