chore: add formatting job to GitHub Actions

Author: 8bitbuddhist

.github/workflows/test.yml

@@ -5,7 +5,14 @@ on:
     branches:
       - master
 jobs:
+  nixfmt:
+    runs-on: ubuntu-latest
+    steps:
+    - uses: actions/checkout@v4
+    - uses: cachix/install-nix-action@v31
+    - run: nix fmt .
   tests:
+    needs: nixfmt
     runs-on: ubuntu-latest
     steps:
     - uses: actions/checkout@v4