ci: replace Cirrus CI with GitHub Actions

Migrate all CI tasks from Cirrus CI (.cirrus.yml) to a new GitHub Actions
workflow (.github/workflows/ci.yml). All tests run with the same conditions.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
Signed-off-by: Giuseppe Scrivano <gscrivan@redhat.com>

Author: giuseppe