diff_test relies on a diff binary being installed on the host

[UebelAndre]

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.