Merge pull request #6188 from ethereum-optimism/willc/only-pnpm

feat: Add script to stop people from accidentally using yarn install

package.json

@@ -38,6 +38,7 @@
     "lint:specs:fix": "npx markdownlint-cli2-fix \"./specs/**/*.md\"",
     "lint:specs:check": "npx markdownlint-cli2  \"./specs/**/*.md\"",
     "lint:specs:toc": "npx doctoc '--title=**Table of Contents**' ./specs",
+    "preinstall": "npx only-allow pnpm",
     "postinstall": "patch-package && (test -d docs/op-stack && cd docs/op-stack && npx yarn@1 install && cd ../.. || exit 0)",
     "ready": "pnpm lint && pnpm test",
     "prepare": "husky install",