Merge pull request #36460 from laravel/fix-build

[6.x] Use ubuntu-18.04

Author: driesvints

.github/workflows/tests.yml

@@ -8,7 +8,7 @@ on:
 
 jobs:
   linux_tests:
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-18.04
 
     services:
       memcached: