[FR]: Allow diff_test to specify diff toolchain

### 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).

Start with the diff_test and write_source_files entry points, tracing how they currently resolve diff from PATH. Determine how a toolchain or label could select the binary, and consider the NixOS case; the work is done when callers can provide a diff binary without requiring diff to be preinstalled.

Written by the indexing model from the issue text.