push.yml: run Linux jobs on "ubuntu-slim" runners

Author: stephengold

.github/workflows/push.yml

@@ -7,7 +7,7 @@ on: [push]
 jobs:
   Java17-Linux:
     if: contains(toJson(github.event.commits), '[ci skip] ') == false
-    runs-on: ubuntu-24.04
+    runs-on: ubuntu-slim
     steps:
       - uses: actions/setup-java@v5
         with: