`diff_test` relies on a `diff` binary being installed on the host

If `diff` is not installed on the host and `PATH` is not configured for the action to find it then the test fails. Ideally there would be some `diff_test_toolchain` rule that would allow users to register either Label or a custom string to where `diff` might be found. If this were implemented with toolchains, the `diff_test` rule could optionally depend on it and fallback to using `diff` just as it does today.