Add env attr to native_binary and native_test

native_binary and native_test currently have no way to specify the environment that the target runs with. A `RunEnvironmentInfo` provider could be returned from these, based on a passed-in `env` attr.

Start by locating the definitions of native_binary and native_test in bazel-skylib, then read the Bazel documentation for RunEnvironmentInfo and existing environment-related providers. Determine how a passed-in env attribute should produce the provider for both rules; done means both targets expose the intended runtime environment and the relevant tests pass.

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