AtlasBuildScripts: Handle existing tags in tag_build.sh
Improvements to tag_build.sh
used in the nightly builds. If a given
tag already exists, instead of failing, do a checkout of that tag. This
will allow the build of multiple projects/platforms from the same
nightly tag.
Relates to ATLINFR-3334, cc @akraszna @aundrus
This script is only used in our nightly builds and will have no impact on the CI.