Use cross-env in build scripts

Closes GH-2006.
Closes GH-2007.

Reviewed-by: Titus Wormer <tituswormer@gmail.com>

package.json

@@ -39,6 +39,7 @@
     "c8": "^7.0.0",
     "capture-website": "^2.0.0",
     "chrome-aws-lambda": "^10.1.0",
+    "cross-env": "^7.0.3",
     "cssnano": "^5.0.0",
     "dlv": "^1.0.0",
     "esbuild": "^0.14.0",
@@ -120,10 +121,10 @@
     "prepare": "husky install",
     "clean": "npm exec -c \"rimraf node_modules\" --workspaces",
     "docs-prep": "node --unhandled-rejections=strict website/prep.js && postcss docs/_asset/index.css -o public/index.css",
-    "docs-bundle-dev": "NODE_ENV=development node --unhandled-rejections=strict website/bundle.js",
-    "docs-bundle-prod": "NODE_ENV=production node --unhandled-rejections=strict website/bundle.js",
-    "docs-generate": "NODE_LOADER_CONFIG=website/loader-react-server.js node --unhandled-rejections=strict --no-warnings --experimental-loader @node-loader/core --conditions=react-server website/generate.server.js",
-    "docs-prerender": "NODE_LOADER_CONFIG=website/loader.js node --unhandled-rejections=strict --no-warnings --experimental-loader @node-loader/core website/prerender.js",
+    "docs-bundle-dev": "cross-env NODE_ENV=development node --unhandled-rejections=strict website/bundle.js",
+    "docs-bundle-prod": "cross-env NODE_ENV=production node --unhandled-rejections=strict website/bundle.js",
+    "docs-generate": "cross-env NODE_LOADER_CONFIG=website/loader-react-server.js node --unhandled-rejections=strict --no-warnings --experimental-loader @node-loader/core --conditions=react-server website/generate.server.js",
+    "docs-prerender": "cross-env NODE_LOADER_CONFIG=website/loader.js node --unhandled-rejections=strict --no-warnings --experimental-loader @node-loader/core website/prerender.js",
     "docs-post": "node --unhandled-rejections=strict website/post.js",
     "docs-dev": "npm run docs-prep && npm run docs-bundle-dev && npm run docs-generate && npm run docs-prerender && npm run docs-post",
     "docs-prod": "npm run docs-prep && npm run docs-bundle-prod && npm run docs-generate && npm run docs-prerender && npm run docs-post",