ci: restrict Lango stale PRs workflow

If one uses mail notifications about failed ci workflows, he can see
tons of the following notifications for his fork of Tarantool:
| [Buristan/tarantool] Run failed: lango-stale-reviews - master
This job failed due to unknown credentials.

This patch adds the condition to run this workflow only for main
Tarantool repo.

NO_DOC=Workflow fix
NO_TEST=Workflow fix
NO_CHANGELOG=Workflow fix