switch docker image to ubuntu-latest and test actions flow

.gitea/workflows/update-flake-lock.yaml

@@ -3,8 +3,13 @@ on: [push]
 
 jobs:
   lockfile:
-    runs-on: nix
+    runs-on: ubuntu-latest
     steps:
-      - run: echo ${{gitea}}
-      - run: echo ${{env}}
-
+      - name: Checkout repository
+        uses: actions/checkout@v4
+      - name: Install Nix
+        uses: DeterminateSystems/nix-installer-action@main
+      - name: Update flake.lock
+        uses: DeterminateSystems/update-flake-lock@main
+        with:
+          pr-title: "Update flake.lock" # Title of PR to be created