Expand "Make" variables inside of values in the env dictionary on test rules

[aaronsky]

This PR implements a gap introduced when bazelbuild/rules_apple#2476 was introduced, where expandable env settings on test rules would not be expanded into the xcscheme during generation. There are a couple of things that reviewers should be aware of before proceeding with this change:

• This change is not considerate of the rules_apple being used, meaning that it assumes the developer is using at least version 3.6.0 (when the breaking changes in Fix label encoding in incremental generators #2476 were introduced). This is not good and should be considered a blocker, but I don't know how to address this with the tools offered by the ruleset or Bazel.
• This change as-implemented requires regeneration of the xcschemes if the configured value of the "Make" variable ever changes.