ci: fix Binaryen cache on Windows

The wrong path was used to cache binaryen, so it wasn't actually getting
cached. Therefore, wasm-opt was rebuilt on every new PR (slowing down
the "Build TinyGo release tarball" a lot).

.github/workflows/windows.yml

@@ -66,8 +66,8 @@ jobs:
         uses: actions/cache@v2
         id: cache-binaryen
         with:
-          key: binaryen-v1
-          path: build/binaryen
+          key: binaryen-windows-v2
+          path: build/wasm-opt.exe
       - name: Build Binaryen
         if: steps.cache-binaryen.outputs.cache-hit != 'true'
         run: make binaryen