Add environment variables to BuildSettings

[shoo]

This change is to specify environment variables when running various commands for preparation and cleanup, building, testing, and so on.
Environment variables can be specified both in project-specific settings and in dub settings.

This change will fix the following two issues:

• How to assign an environment variable from DUB? #940
• Package descriptor attribute commandsEnvVariables #1885

And related:

• Support additional compiler path search other than the PATH env var. #269
• support environment variables in more places #767
• Set dub settings via environment variables #1745
• https://github.com/dlang/dub/issues?page=1&q=is%3Aissue+is%3Aopen+environment

@andre2007 It seems to be related to some of the issues you suggested. I would appreciate your opinion.

[dlang-bot]

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[andre2007]

@shoo Thanks a lot for working on this. The PR looks really solid and complete. I can't see any obvious issues or missing things.

[shoo]

Is there anyone who can review this PR?
Or can you recommend a suitable reviewer?