feat: enable gitea actions in forgejo

See: https://docs.gitea.com/usage/actions/quickstart

.forgejo/workflows/check.yml

@@ -0,0 +1,12 @@
+name: Flake checks
+on: [push]
+
+jobs:
+  Check:
+    runs-on: nix-flakes
+    steps:
+      - name: Check out repository code
+        uses: https://code.forgejo.org/actions/checkout@v4
+      - name: Run flake checks
+        run: |
+          nix --print-build-logs --verbose --accept-flake-config flake check

hosts/nachtigall/apps/forgejo.nix

@@ -78,6 +78,11 @@
       session = {
         COOKIE_SECURE = lib.mkForce true;
       };
+      # See https://forgejo.org/docs/latest/admin/actions/
+      actions.ENABLED = true;
+      # In an actions workflow, when uses: does not specify an absolute URL,
+      # the value of DEFAULT_ACTIONS_URL is prepended to it.
+      actions.DEFAULT_ACTIONS_URL = "https://code.forgejo.org";
     };
   };