ci: use ubuntu 18 specifically (#24904)

diff --git a/.github/workflows/ci-tests.yml b/.github/workflows/ci-tests.yml
index 3f7d4bb..78c2f5a 100644
--- a/.github/workflows/ci-tests.yml
+++ b/.github/workflows/ci-tests.yml
@@ -4,7 +4,7 @@
 
 jobs:
   test:
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-18.04
 
     strategy:
       fail-fast: false