chore(workflows): add-pr-to-project-board only trigger on `opened`

[0x-r4bbit]

This is to avoid getting these failing CI tasks where adding a PR to the board fails when it was already added before (happens when pushing into an existing PR).

Checklist

Ensure you completed all of the steps below before submitting your pull request:

• [ ] Added natspec comments?
• [ ] Ran forge snapshot?
• [x] Ran pnpm lint?
• [ ] Ran forge test?
• [ ] Ran pnpm verify?