feat: Add test job in Nix workflow

- A new test job is added in the Nix Github Actions workflow
- This test runs on ubuntu, and checks out the current repository and lists its content
- These steps were added to increase visibility on the test environment

.forgejo/workflows/nix.yaml

@@ -1,5 +1,13 @@
 on: [push]
 jobs:
+  test:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v3
+      - run: |
+          pwd
+          ls -al
+
   build:
     runs-on: nix
     steps: