Fix branch name determination in TrigUpgradeTest post-exec

Same fix as !21757 (merged), but for TrigUpgradeTest.

I realised the env variable AtlasBuildBranch is not available in CI, which makes the rootcomp step complain about a missing reference. This MR makes TrigUpgradeTest use gitlabTargetBranch if AtlasBuildBranch is not available.

This will make the rootcomp step run correctly in the CI, even though the CI result does not depend on rootcomp result (but you can still read the logs).

Related JIRA: ATR-19383

Edited by Rafal Bielski

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