The previous logic used GITHUB_HEAD_REF, which on a pull request is
the source (PR author's) branch name. For backport PRs (e.g.
mrpollo/backport-26781-1.17), no matching branch exists in
px4-ros2-interface-lib, so the script fell back to main and the
build broke from uORB message divergence.
Switch to GITHUB_BASE_REF, which on a PR is the branch the code is
being merged into (main or release/X.Y), and fall back to
GITHUB_REF_NAME for direct pushes. This always resolves to a real
branch in px4-ros2-interface-lib.
Signed-off-by: Ramon Roche <mrpollo@gmail.com>
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