RFE: support substitution in environment.environment

[smoser]

I have a situation where I would like to put SETUPTOOLS_SCM_PRETEND_VERSION=${{package.version}} into the build environment.

I want to do:

environment:
  environment:
    SETUPTOOLS_SCM_PRETEND_VERSION: ${{package.version}}
    OTHER: "other is ${{vars.VARS}}"

The second 'OTHER' there i just put there. I think it is reasonable to support substitution within a string rather than only supporting the entire string. but I do not want to request shell-like behavior or expansion.

I also would lke to request that work within a 'range' section like seen here.