ci(actions): support triggering a build with PR comment

Author: jrolfs

.github/workflows/build.yml

@@ -8,11 +8,16 @@ on:
       - next
       - alpha
       - beta
+  issue_comment:
+    types: [created]
 
 jobs:
   build:
     name: Build + Test + Release
     runs-on: ubuntu-latest
+    if:
+      ${{ github.event_name == 'pull_request' || github.event_name == 'push' ||
+      contains(github.event.comment, 'rebuild') }}
 
     steps:
       - name: Get Yarn cache path