Run slash command dispatch workflow on Pull Request comments only (#1248)

The workflow ran on every GitHub issue and pull request
comments. This patch ensures that the the slash command
dispatch workflow is triggered by Pull Request comments
only. Patches #646.

Author: weiji14

.github/workflows/slash-command-dispatch.yml

@@ -6,6 +6,7 @@ on:
     # types: [created, edited]
 jobs:
   slashCommandDispatch:
+    if: ${{ github.event.issue.pull_request }}
     runs-on: ubuntu-latest
     steps:
       # Generate token from GenericMappingTools bot