Allow manually triggered publishes

This allows AssemblyScript#2786 (which fixes the regression from AssemblyScript#2757) to be
published as soon as possible. Note that the main branch is the only
branch that can be published, since `ref: main` is passed to
actions/checkout.

.github/workflows/publish.yml

@@ -1,5 +1,8 @@
 name: Publish
 on:
+  # Note that the main branch will be used regardless of the branch chosen
+  # in the web interface.
+  workflow_dispatch:
   schedule:
   - cron: '0 0 * * *'
 jobs: