Update cross-env requirement from ^5.2.1 to ^6.0.0

Updates the requirements on [cross-env](https://github.com/kentcdodds/cross-env) to permit the latest version.
- [Release notes](https://github.com/kentcdodds/cross-env/releases)
- [Changelog](https://github.com/kentcdodds/cross-env/blob/master/CHANGELOG.md)
- [Commits](https://github.com/kentcdodds/cross-env/compare/v5.2.1...v6.0.0)

Signed-off-by: dependabot-preview[bot] <support@dependabot.com>

package.json

@@ -41,7 +41,7 @@
     "chai": "^4.2.0",
     "chalk": "^2.4.2",
     "cpy": "^7.3.0",
-    "cross-env": "^5.2.1",
+    "cross-env": "^6.0.0",
     "cssnano": "^4.1.10",
     "del": "^5.1.0",
     "eslint": "^6.3.0",