Make `package-and-qa` always available when targeting a stable branch

We want to be able to run package-and-qa on patch release preparation merge requests.

For now this has to be manual because otherwise we can't set OMNIBUS_BRANCH properly, however after !83602 (merged) OMNIBUS_BRANCH will be set correctly automatically, so if no other variables we want to fill manually, this can be automated.

Context: gitlab-org/release/tasks#3641 (comment 891389231)