ci: add diffutils to Windows MSYS2 setup

Author: tydeu

Diff for: .github/workflows/ci.yml

@@ -42,7 +42,7 @@ jobs:
       uses: msys2/setup-msys2@v2
       with:
         path-type: inherit
-        install: curl unzip make mingw-w64-x86_64-gcc mingw-w64-x86_64-gmp
+        install: curl unzip make diffutils mingw-w64-x86_64-gcc mingw-w64-x86_64-gmp
     - name: Install Elan (Windows)
       if: matrix.os == 'windows-latest'
       run: |