ci: still use Ubuntu 20.04 for cross test

.github/workflows/ci.yml

@@ -73,7 +73,10 @@ jobs:
   # Use cross for QEMU-based testing
   # cross needs to execute Docker, GitHub Action already has it installed
   cross:
-    runs-on: ubuntu-24.04
+    # Still use 20.04 for this CI step as test `test_prctl::test_set_vma_anon_name`
+    # would fail on 22.04 and 24.04 (at least for now)
+    # https://github.com/nix-rust/nix/issues/2418
+    runs-on: ubuntu-20.04
     needs: [rustfmt, minver, macos, linux_native_builds, rust_stable]
     strategy:
       fail-fast: false