2dfdr pipeline fix

rds/acmmSystem!1 caused 2dfdr_ac pipeline to fail (see https://dev.aao.org.au/rds/2dfdr/2dfdr_ac/-/jobs/1286), whilst it was running aaoBuildFetch, fetching the 2dfdr_ac folder itself (not submodules). This is due to rds/acmmSystem!1 introducing a change that made the aaoBuildFetch script attempt to fetch the origin and run a git remote set-head origin -a. This failed on the 2dfdr_ac pipeline, which meant there was no origin/HEAD, which caused the script to fail fatally.

This fix works around the issue by changing origin/HEAD to just HEAD if origin/HEAD fails to be created. I have not yet determined why git remote set-head origin -a fails on the 2dfdr_ac pipeline.