Triggering build with tag triggers branch instead of tag build

[jerr0328]

Describe the bug
When manually triggering a build with a tag on master, Infrabox ran a master branch build instead of a tag build, where the tag was not in the environment variable.

To Reproduce
Steps to reproduce the behavior:

0. It's likely that we saw this when we had an outage during the initial tagging, so assume that a tag happened while github hooks weren't working. I have not verified that this happens all the time or not.
1. Go to the project where you have a tag
2. Click on "Trigger Build"
3. Put the tag name in "Branch or Sha"
4. Notice that the build is running for the master branch.

Expected behavior
I would expect a tag build to be run with that tag ref, as I would need to trigger a tag build if the webhook failed.

Additional context
Using Infrabox build_1859