Update dependency cross-env to v6

Author: renovate-bot

package.json

@@ -35,7 +35,7 @@
     "@types/ultra-strftime": "^1.0.0",
     "chai": "^4.2.0",
     "coveralls": "^3.0.6",
-    "cross-env": "^5.2.1",
+    "cross-env": "^6.0.3",
     "dirty-chai": "^2.0.1",
     "eslint": "^6.5.1",
     "eslint-config-prettier": "^6.4.0",