Add a new lint to check for use of std::env::set_var in tests

[jamadeo]

Adds a new lint to check for use of std::env::set_var in unit tests.

In our experience in https://github.com/block/goose, setting environment variables in tests has been the largest source of test flakiness. Test authors often think that if they properly clean up the mutated variable, all is well. But because tests run concurrently, there is really no way to isolate this effect other than marking them using e.g. https://crates.io/crates/serial_test.

changelog: [set_env_in_tests]: checks for use of std::env::set_var in unit tests

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