Update CI to use nix runner

.forgejo/workflows/build.yaml

@@ -1,18 +1,16 @@
+# SPDX-FileCopyrightText: 2024 Simon Bruder <simon@sbruder.de>
+#
+# SPDX-License-Identifier: AGPL-3.0-or-later
+
 on: [push]
 jobs:
   build:
-    runs-on: docker
+    runs-on: nix
     steps:
-      - name: Set up dependencies
-        run: |
-          apt-get update
-          apt-get -y install sudo
-      - name: Install Nix
-        uses: https://github.com/cachix/install-nix-action@v27
       - name: Checkout
         uses: actions/checkout@v4
       - name: Build
-        run: nix build -L
+        run: nix build -L .#
       - name: Upload
         uses: actions/upload-artifact@v3
         with: