ci: workaround release rockspec publishing problem

The problem is described in [1]: `tarantoolctl rocks` from tarantool
1.10 doesn't work on Ubuntu Jammy, when it is installed by the
setup-tarantool GitHub Action. Let's use Ubuntu Focal as a temporary
workaround.

[1]: tarantool/setup-tarantool#36

Author: Totktonada

.github/workflows/publish.yml

@@ -29,7 +29,7 @@ jobs:
   publish-tag:
     if: startsWith(github.ref, 'refs/tags/')
     needs: version-check
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-20.04
     steps:
       - uses: actions/checkout@v3