ci: add forgejo pipeline with nix flake check and build

.forgejo/workflows/ci.yml

@@ -0,0 +1,27 @@
+name: Continuous Integration
+
+on:
+  push:
+  workflow_dispatch:
+
+jobs:
+  check:
+    name: Lint, Check & Test
+    runs-on: nix
+
+    steps:
+      - name: Check out Repository
+        uses: https://git.konstantinfickel.de/actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd # v6
+
+      - run: nix --version
+      - run: nix flake check
+
+  build:
+    name: Build Package
+    runs-on: nix
+
+    steps:
+      - name: Check out Repository
+        uses: https://git.konstantinfickel.de/actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd # v6
+
+      - run: nix build