🤖 config(package.json): Use custom node environment in coverage script.

These changes were automatically generated by a transform whose code can be found at:
  - https://github.com/make-github-pseudonymous-again/rejuvenate/blob/b30cf223897adb74ca4a854799d50b21d7e1959d/src/transforms/coverage:use-custom-node-env.js
Please contact the author of the transform if you believe there was an error.

package.json

@@ -42,7 +42,7 @@
     "build-docs": "esdoc",
     "build-gh-pages": "npm run build-docs",
     "commit-msg": "commitlint --edit",
-    "cover": "c8 --all --src src --reporter=lcov npm test",
+    "cover": "NODE_ENV=cover c8 --all --src src --reporter=lcov npm test",
     "dev": "npm run lint -- --fix && npm run cover -- -- -st --fail-fast",
     "install-hooks": "husky install",
     "lint": "true",