Introduce EMBROIDER_WORKING_DIRECTORY env var for concurrent builds

[simonihmig]

Currently Embroider will write rewritten packages and other build meta data into node_modules/.embroider. That location is hardcoded. Usually not a problem, but when running more than one build for the same app concurrently, builds will step on each others toes by trying to write into the same dir. The optional environment variable introduced here will allow to specify that directory for each build to isolate them.

Fixes #2086

[simonihmig]

Do we want to document this somewhere, or is this more like a hidden feature? t seems no one than us had a need for this before...

Also no tests. I could add some unit tests for the locateEmbroiderWorkingDir, which does not have any yet. E2E testing the actual "feature" of concurrent builds in CI seems like too much overhead I think? I tested this locally, and it was solving our problem.

[SergeAstapov]

+1 for documenting this.

as for question "where to document this", maybe "Options" section in readme.md would work

[mansona]

I feel like this probably should have a test for it 🙈 it seems like a sensible change if we can guarantee that everything is going through this function that relies on that location, but verifying would make me feel more comfortable about that. Also it means that we would have a regression test to avoid breaking this in future

[ef4]

I agree that code not using locateEmbroiderWorkingDir is a risk, but I don't think it's particularly practical to guard against with a test. For the test to help you need to remember that your example needs to work under the env var, and if you remember that you could just have used locateEmbroiderWorkingDir anyway.

I'm thinking specifically of template-tag-codemod, which does in fact currently have this bug and would be unlikely to be caught by any general test here.