Fix webpacker thread safety

[ahukkanen]

🎩 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.

[microstudi]

thanks Antti!

[andreslucena]

👍🏽👍🏽👍🏽
I've tried the spec alone on develop and also on ruby-3.0 branches, on both it failed, except for one time when it passed.
Then I've checked this fix on both branches, on both it worked. Just to be extra sure, I've rerun it 50 times, as I had the flaky pass before. On all the 100 runs, it passed (OK/green).
I checked the admin's newsletter templates page on ruby-3.0 and it worked too.