tests: use env-lock to more safely modify env variables

[intgr]

env-lock:

• Restores old environment after the test ends.
• Locks access to environment variables.

Environment access during modification on Unixes is inherently racy: https://doc.rust-lang.org/std/env/fn.set_var.html

This still doesn't protect other tests that may read environment variables, but it's good enough for tests.