Update to event-driven TagBot.yml (#850)

As documented at https://discourse.julialang.org/t/ann-required-updates-to-tagbot-yml/49249

Author: c42f

.github/workflows/TagBot.yml

@@ -1,13 +1,14 @@
 name: TagBot
 on:
-  schedule:
-    - cron: 0 * * * *
+  issue_comment:
+    types:
+      - created
+  workflow_dispatch:
 jobs:
   TagBot:
+    if: github.event_name == 'workflow_dispatch' || github.actor == 'JuliaTagBot'
     runs-on: ubuntu-latest
     steps:
       - uses: JuliaRegistries/TagBot@v1
         with:
           token: ${{ secrets.GITHUB_TOKEN }}
-          # Workaround for https://github.com/JuliaDocs/Documenter.jl/issues/1177
-          ssh: ${{ secrets.DOCUMENTER_KEY }}