Allow abstracting over process.env

[mbg]

Two straight-forward commits cherry-picked from #3990 (and, in turn, #3973) which add an Env interface that allows us to abstract over process.env so that tests don't have to run synchronously because they query process.env and can instead query a test-specific environment.

Not used in this PR, but cherry-picked so that we can hopefully merge it relatively quickly and make use of it elsewhere.

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