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

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).

Author: 0x-r4bbit

.github/workflows/add-pr-to-project-board.yml

@@ -2,6 +2,8 @@ name: Add PR to task board
 
 on:
   pull_request:
+    types:
+      - opened
 
 jobs:
   add-to-project: