Add initial nix building github actions

To reduce the amount of redudand rebuilds cachix is used to store
outputs. The cachix cache should be accessible in the cachix UI to
everyone in the @chaos-jetzt/infra team

.github/workflows/build-systems.yaml

@@ -0,0 +1,55 @@
+name: "Build system config"
+on:
+  pull_request:
+  push:
+jobs:
+  avoid_duplicates:
+    runs-on: ubuntu-latest
+    outputs:
+      should_skip: ${{ steps.skip_check.outputs.should_skip }}
+    steps:
+      - id: skip_check
+        uses: fkirc/skip-duplicate-actions@v5
+        with:
+          # All of these options are optional, so you can remove them if you are happy with the defaults
+          concurrent_skipping: 'same_content_newer'
+          skip_after_successful_duplicate: 'true'
+          do_not_skip: '["workflow_dispatch", "schedule"]'
+  build:
+    runs-on: ubuntu-latest
+    needs: avoid_duplicates
+    if: needs.avoid_duplicates.outputs.should_skip != 'true'
+    steps:
+    - uses: actions/checkout@v3
+    - uses: cachix/install-nix-action@v19
+      with:
+        github_access_token: ${{ secrets.GITHUB_TOKEN }}
+    - uses: cachix/cachix-action@v12
+      with:
+        name: chaos-jetzt-nixfiles
+        authToken: '${{ secrets.CACHIX_AUTH_TOKEN }}'
+    - name: nix flake check
+      run: |
+        nix flake check --no-build
+        echo "## \`nix flake check\` succeeded" >> $GITHUB_STEP_SUMMARY
+        echo "" >> $GITHUB_STEP_SUMMARY
+    - name: Discover nixosConfigurations
+      run: |
+        echo Discovered the following system configs
+        nix flake show . --json | jq ".nixosConfigurations|keys[]" -r
+    - name: Build systems
+      run: |
+        echo "## Builds succeeded" >> $GITHUB_STEP_SUMMARY
+        echo "" >> $GITHUB_STEP_SUMMARY
+        echo "| Host | Out path |" >> $GITHUB_STEP_SUMMARY
+        echo "| ---- | -------- |" >> $GITHUB_STEP_SUMMARY
+        for host in $(nix flake show . --json | jq ".nixosConfigurations|keys[]" -r); do
+          echo "::group::Building ${host}"
+          drv=".#nixosConfigurations.$host.config.system.build.toplevel"
+          build_cmd="nix build ${drv}"
+          cachix watch-exec chaos-jetzt-nixfiles -- $build_cmd
+          echo "::endgroup::"
+          out_path=$($build_cmd --print-out-paths)
+          echo -e "\x1b[32;1mSuccessfully built .#nixosConfigurations.${host}\x1b[0m"
+          echo "| ${host} | \`${out_path}\` |" >> $GITHUB_STEP_SUMMARY
+        done