TrigValSteering.CheckSteps: Improve reference file path building
Neither of the two variables AtlasBuildBranch and gitlabTargetBranch is available in Jenkins job building the nightly release and running the unit tests. This case was badly handled by the reference file path building in CheckSteps and caused unit test failures. Add another case using third variable JOB_NAME
and improve the error handling in case all variables are missing.
Fixes ATR-21836 (the unit test failures in nightly builds)