[3.8] Pin test runner to Ubuntu 18 to un-break CI (GH-24715) (GH-24717)

(cherry picked from commit 94894dd45e4bad6efb27eac4497b24cdc18b3e2d)

Co-authored-by: Brandt Bucher <brandt@python.org>
Automerge-Triggered-By: GH:tiran

diff --git a/.github/workflows/build.yml b/.github/workflows/build.yml
index 669c541dd4f49b8b6b34c678c350f92ff68c455c..5822428e34a862d3cfc94f5dc0937b89624430d0 100644 (file)
--- a/.github/workflows/build.yml
+++ b/.github/workflows/build.yml
@@ -114,7 +114,7 @@ jobs:
 
   build_ubuntu:
     name: 'Ubuntu'
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-18.04
     needs: check_source
     if: needs.check_source.outputs.run_tests == 'true'
     env: