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

[aaronsky]

Fixes #3056

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.

[brentleyjones]

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.

I'm fine with bumping our minimum required version of rules_apple to 3.6.0. I agree there is no good way to detect this otherwise. Though, we could add a flag to go back to the old behavior? I would rather just bump our dep though.

This change as-implemented requires regeneration of the xcschemes if the configured value of the "Make" variable ever changes.

I don't think there is a better way?

[aaronsky]

I've increasingly come to regret the way I introduced support for this feature in rules_apple. I think the most flexible behavior from a UX standpoint is a way to map Bazel settings to project-level xcconfigs, so they're available to the scheme without having to regenerate them, but then you'd need to regenerate the xcconfig instead.

I can put up a new commit that bumps the minimum rules_apple to 3.6.0. Thank you for taking a look at this draft.