ci: run zig-test on ubuntu-20.04

Author: krichprollsch

.github/workflows/zig-test.yml

@@ -100,7 +100,7 @@ jobs:
     # Don't run the CI with draft PR.
     if: github.event.pull_request.draft == false
 
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-20.04
     container:
       image: ghcr.io/lightpanda-io/zig-v8:0.12.0-dev.1773-8a8fd47d2
       credentials: