Feature request: Support using environment variables in the definition of variables in EnvVars section

[hagertnl]

Currently, in rgt_test_input.ini, a line like:

[EnvVars]
MY_SCRATCH = $HOME/Scratch

results in $HOME not being expanded, and is equivalent to export MY_SCRATCH='$HOME/Scratch'.

It would be very useful to run the definition of environment variables through an interpreter to automatically replace some of these. I think that the harness should emit a warning and leave the variable uninterpreted if the variable is not found in the environment.

[hagertnl]

Might benefit from: https://docs.python.org/3/library/configparser.html#configparser.ExtendedInterpolation