🎩 What? Why?

We noticed at #8452 that Webpacker is not thread safe which is hopefully the last blocker for the Ruby 3 upgrade.

This should fix Webpacker thread safety according to the discussion and instructions at:
rails/webpacker#2801

There is a further fix for determining the full expanded paths inspired by this commit at Shakapacker:
shakacode/shakapacker@f2dc437

📌 Related Issues

• Related to Update ruby to 3.0 #8452, warning: conflicting chdir during another chdir block rails/webpacker#2801

Testing

• Ruby 2.7: Run the introduced spec and see no warnings in console.
• Ruby 3.0: Run the introduced spec and see no exceptions in the console

📋 Checklist

• ❓ CONSIDER adding a unit test if your PR resolves an issue.
• ✔️ DO check open PR's to avoid duplicates.
• ✔️ DO keep pull requests small so they can be easily reviewed.
• ✔️ DO build locally before pushing.
• ✔️ DO make sure tests pass.
• ✔️ DO make sure any new changes are documented in docs/.
• ✔️ DO add and modify seeds if necessary.
• ✔️ DO add CHANGELOG upgrade notes if required.
• ✔️ DO add to GraphQL API if there are new public fields.
• ✔️ DO add link to MetaDecidim if it's a new feature.
• ❌AVOID breaking the continuous integration build.
• ❌AVOID making significant changes to the overall architecture.