[Python-checkins] Pin test runner to Ubuntu 18 to un-break CI (#24715)

[nascheme]

https://github.com/python/cpython/commit/94894dd45e4bad6efb27eac4497b24cdc18b3e2d
commit: 94894dd45e4bad6efb27eac4497b24cdc18b3e2d
branch: master
author: Brandt Bucher <brandt at python.org>
committer: nascheme <nas-github at arctrix.com>
date: 2021-03-02T23:03:55-08:00
summary:

Pin test runner to Ubuntu 18 to un-break CI (#24715)

files:
M .github/workflows/build.yml

diff --git a/.github/workflows/build.yml b/.github/workflows/build.yml
index c674c5d606c54..ef54865ed964c 100644
--- a/.github/workflows/build.yml
+++ b/.github/workflows/build.yml
@@ -127,7 +127,7 @@ 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: