rustPlatform: fix bug with ambiguous diff tool

If a user provides `nativeBuildInputs = [ llvmPackages.bintools ]` or any other
package containing a `${prefix}/bin/diff`, the builder could use it instead
of the standard unix `diff`, causing a build failure.

This updates the call to specify an abspath to `diff` and avoid reliance on `PATH`.

Resolves #87081