build: drop diff.exe from windows builds

The shipped versions of xdiff already does everything diff does, so this
duplication of tools isn't necessary. Furthermore, this setup is more
consistent overall, as the 'diffopt=external' option should be for
external programs rather than programs we bundle neovim with.

Install diffutils for oldtests in CI to avoid needing to modify tests.

.github/workflows/test.yml

@@ -354,7 +354,7 @@ jobs:
         with:
           update: true
           pacboy: >-
-            make:p gcc:p
+            make:p gcc:p diffutils:p
           release: false
 
       - if: success() || failure() && steps.abort_job.outputs.status == 'success'