workflows: Run nightly on ubuntu-latest

There is nothing complicated going on here (no containers, no large tar
piping, etc.). Our bots tend to use modern Python, so always stay on the
latest Ubuntu release instead of pinning down and old release.

.github/workflows/nightly.yml

@@ -8,7 +8,7 @@ jobs:
   trigger:
     permissions:
       statuses: write
-    runs-on: ubuntu-22.04
+    runs-on: ubuntu-latest
     steps:
       - name: Clone repository
         uses: actions/checkout@v4