ci: pin runner version to Ubuntu 20.04

Now the `ubuntu-latest` label points to Ubuntu 22.04 instead of 20.04.
Due to this fact, the testing workflow started to fail with an error
like this:

    Version 3.5 was not found in the local cache
    Error: Version 3.5 with arch x64 not found
    The list of all available versions can be found here:
    https://raw.githubusercontent.com/actions/python-versions/main/versions-manifest.json

Pin the runner version to Ubuntu 20.04 to avoid the error above.

.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-latest
+    runs-on: ubuntu-20.04
 
     strategy:
       fail-fast: false