* Suppress `-Wdeprecated-copy` on Clang 13
Clang 13 now also emits `-Wdeprecated-copy` when a copy assign operator is deleted. Since we need both `actor` to be an aggregate and to disallow assignment -- the only other way is to suppress the warning.
* Fix CI: replace `os: "ubuntu-xx.04"` workers with containers
ubuntu-16.04 is already removed from GHA and ubuntu-18.04 will be soon removed too
* Fix CI: Install only necessary stuff in containers
* Run CI in any branch