ci: simplify debian 12 check

.github/workflows/test.yml

@@ -840,9 +840,6 @@ jobs:
     runs-on: namespace-profile-ghostty-sm
     needs: [test, build-dist]
     steps:
-      - name: Checkout code
-        uses: actions/checkout@v4
-
       - name: Install and configure Namespace CLI
         uses: namespacelabs/nscloud-setup@v0
 
@@ -854,10 +851,15 @@ jobs:
         with:
           name: source-tarball
 
+      - name: Extract tarball
+        run: |
+          mkdir dist
+          tar --verbose --extract --strip-components 1 --directory dist --file ghostty-source.tar.gz
+
       - name: Build and push
         uses: docker/build-push-action@v6
         with:
-          context: .
-          file: src/build/docker/debian/Dockerfile
+          context: dist
+          file: dist/src/build/docker/debian/Dockerfile
           build-args: |
             DISTRO_VERSION=12