fix: use cssnano v5.0.0-rc.1, work around a npm 6 hoisting bug

Fixes vuejs#6342

Author: haoqunjiang

packages/@vue/cli-service/package.json

@@ -47,7 +47,7 @@
     "copy-webpack-plugin": "^7.0.0",
     "css-loader": "^5.1.1",
     "css-minimizer-webpack-plugin": "^1.2.0",
-    "cssnano": "^4.1.10",
+    "cssnano": "^5.0.0-rc.1",
     "debug": "^4.1.1",
     "default-gateway": "^6.0.3",
     "dotenv": "^8.2.0",