ci: Package on the omnibus branch matching the MR target

What does this MR do?

build-package-and-qa triggers the omnibus-gitlab-mirror pipeline without a branch, so it always runs against omnibus master. For an MR targeting a stable branch, that packages the stable Gitaly on master's builder image instead of the matching stable builder.

The builders can use different Go toolchains. A stable Gitaly built for Go 1.26 (GOFIPS140=off, boringcrypto) fails on the master builder once it moves to Go 1.27 (native FIPS):

imports gitlab.com/gitlab-org/labkit/fips
imports crypto/boring: build constraints exclude all Go files in /usr/local/go/src/crypto/boring

This MR passes branch: $OMNIBUS_BRANCH, set from $CI_MERGE_REQUEST_TARGET_BRANCH_NAME on the blocking MR path and defaulting to master. So a 19-3-stable MR packages on omnibus 19-3-stable, and master MRs keep building on master.

Fixes #7534 (closed). To be backported to the active stable branches.

Note

This is not a Gitaly FIPS defect. Native GOFIPS140 toolchains are already supported and verified (#7305 (closed), CNG FIPS pipeline green on 2026-08-20). The fix only ensures a stable-branch MR is packaged on the matching omnibus stable builder (Go 1.26 / boringcrypto) instead of the master builder (Go 1.27 / native), so the toolchain matches what the stable Gitaly is built for.