ci: add woodpecker workflow for checking flake files

Signed-off-by: Christoph Heiss <christoph@c8h4.io>

.woodpecker/check-flake-files.yaml

@@ -0,0 +1,22 @@
+---
+when:
+  - event: push
+  - event: pull_request
+    path:
+      'flakes/**/*.toml'
+
+steps:
+  - name: build
+    image: docker.io/nixos/nix
+    environment:
+      NIX_CONFIG: 'experimental-features = nix-command flakes'
+      RUST_LOG: debug
+    commands:
+      - |
+        for f in flakes/**/*.toml; do
+          nix run --accept-flake-config .#flake-info -- --json group "$f" "$(basename "$f" .toml)" --report
+          if [[ -f report.txt ]]; then
+            cat report.txt
+            exit 1
+          fi
+        done