ci: Package on the omnibus branch matching the MR target (19-3-stable backport)

What does this MR do?

Backport to 19-3-stable of !9490 (merged).

build-package-and-qa triggered the omnibus-gitlab-mirror pipeline without a branch, so a 19-3-stable MR was packaged on omnibus master's builder (Go 1.27 native FIPS) instead of the matching 19-3-stable builder (Go 1.26 boringcrypto), breaking the FIPS build with crypto/boring: build constraints exclude all Go files.

This passes branch: $OMNIBUS_BRANCH (from $CI_MERGE_REQUEST_TARGET_BRANCH_NAME), so MRs to 19-3-stable package on omnibus 19-3-stable.

Fixes #7534 (closed) on this branch.