Upload wpiformat diff if one exists (#3474)

This means users will be able to apply it locally with "git apply".

.github/workflows/lint-format.yml

@@ -31,8 +31,16 @@ jobs:
         run: pip3 install wpiformat
       - name: Run
         run: wpiformat -clang 12
-      - name: Check Output
+      - name: Check output
         run: git --no-pager diff --exit-code HEAD
+      - name: Generate diff
+        run: git diff HEAD > wpiformat-fixes.patch
+        if: ${{ failure() }}
+      - uses: actions/upload-artifact@v2
+        with:
+          name: wpiformat fixes
+          path: wpiformat-fixes.patch
+        if: ${{ failure() }}
   tidy:
     name: "clang-tidy"
     runs-on: ubuntu-latest