Allow env var to always publish

[mbbush]

Description of your changes

Actually do the # TODO(hasheddan): remove master and support overriding from the makefile.

This removes master from the list of branch names that get published, and adds support for an env var, tentatively named DO_PUBLISH, which will cause the build module to publish regardless of branch.

If anyone has a better suggestion for the env var name I'm happy to change it.

Fixes #

I have:

• Read and followed Upbound's contribution process.
• Run make reviewable to ensure this PR is ready for review.
• Added backport release-x.y labels to auto-backport this PR, as appropriate.

How has this code been tested