Feature Request: allow a run input file to define an environment variable

[hagertnl]

I think it could be nice if we allowed the rgt.inp harness input files to define an environment variable.
For example, say you have an application that you know you need to use a specific Git branch for. Then, you could just do a set RGT_GIT_REPS_BRANCH="mybranch" in the rgt.inp file, and you wouldn't have to worry about remembering to set it in your shell beforehand!
We'd have to be careful with the order of things, like if there were multiple set calls on the same environment variable, we'd have to just make sure they're processed and set sequentially.