[FR]: Allow diff_test to specify diff toolchain

[malt3]

What is the current behavior?

Currently, rules like diff_test (and rules that use it, including write_source_files) expect diff to be in the PATH. This fails on NixOS (and platforms where diff is not preinstalled).

Describe the feature

Provide a method to specify the diff binary (via toolchain or label).

[thesayyn]

FWIW, https://uutils.github.io/diffutils/ has an hermetic diffutils binaries, something similar could be done for diff too.