Add vcpkgCache- prefix to cache key

This makes the `key` more consistent with the `buildCachei-` prefixed `key`. This should make it easier to see the purpose of the cache entry on the caches page:

https://github.com/lairworks/nas2d-core/actions/caches

.github/workflows/build.yml

@@ -35,7 +35,7 @@ jobs:
       id: cache
       with:
         path: vcpkg_installed
-        key: ${{ runner.os }}-${{ matrix.platform }}-${{ hashFiles('vcpkg.json') }}
+        key: vcpkgCache-${{ runner.os }}-${{ matrix.platform }}-${{ hashFiles('vcpkg.json') }}
 
     - name: Restore incremental build cache
       uses: actions/cache/restore@v4