Change git version build method to use env vars

[marshallmcdonnell]

Instead of current setup where "mantid-builder" updates the hard-coded Mantid commit SHA in meta yaml header for setting the version of Mantid that is built, we instead switch to using environment variables

This can help remove the hard-coded "nightly" channel in the Jenkins pipeline and upload to specific channels.

This will also help in building/publishing builds from the mantid fork with a separate label in conda.

Report to:

• @peterfpeterson
• @rosswhitfield
• @KedoKudo
• @quantumsteve
• Donnie Earnest
• @mabrahamdevops

Notify:

• @DanNixon

[peterfpeterson]

PR29981 is part of this

[marshallmcdonnell]

@peterfpeterson Is there any update on this ticket? I know you and @mabrahamdevops have been working on making this happen. No rush, thanks!