Run tests on ubuntu-latest

Ubuntu 20.04 is no longer supported by GitHub runners.

Author: locker

.github/workflows/test.yml

@@ -8,7 +8,7 @@ jobs:
         github.event.pull_request.head.repo.full_name != github.repository ) &&
         ( github.repository == 'tarantool/test-run' )
 
-    runs-on: ubuntu-20.04
+    runs-on: ubuntu-latest
 
     strategy:
       fail-fast: false