Set nextstrain.org PRs to manual trigger

This allows us to only create review PRs in the downstream repo when
necessary, reducing the amount of excessive automated PRs.

Note that the title and description of the PR have been updated to use
the source ref (e.g. branch, tag) rather than PR number, since that is
not guaranteed or retrievable with the manual trigger.

.github/workflows/make_prs_for_other_repos.yaml

@@ -1,6 +1,6 @@
 name: "Make PRs for Nextstrain projects which depend on Auspice"
 on:
-  pull_request:
+  workflow_dispatch:
 jobs:
   make-pr-on-nextstrain-dot-org: # <job_id>
     runs-on: ubuntu-latest
@@ -13,29 +13,27 @@ jobs:
         with:
           repository: nextstrain/nextstrain.org
       - name: Install Auspice from PRs HEAD commit
-        if: ${{ github.event_name == 'pull_request' }}
         # Note: $GITHUB_SHA is _not_ the same commit as the HEAD commit on the PR branch
         # see https://github.community/t/github-sha-not-the-same-as-the-triggering-commit/18286/2
         shell: bash
         run: |
-          AUSPICE_COMMIT=$(cat $GITHUB_EVENT_PATH | jq -r .pull_request.head.sha)
+          AUSPICE_COMMIT=$(cat $GITHUB_EVENT_PATH | jq -r .workflow_dispatch.head.sha)
           echo "auspice_commit=$AUSPICE_COMMIT" >> $GITHUB_ENV
           npm ci
           npm install nextstrain/auspice#${AUSPICE_COMMIT}
           git add package.json package-lock.json
       - name: Create Pull Request for testing on nextstrain.org repo
-        if: ${{ github.event_name == 'pull_request' }}
         id: cpr
         uses: peter-evans/create-pull-request@v3
         with:
           token: ${{ secrets.NEXTSTRAIN_BOT_PAT }}
-          branch: "auspice-pr-${{ github.event.pull_request.number }}"
+          branch: "auspice-pr-${{ github.event.workflow_dispatch.ref }}"
           commit-message: "[testing only] upgrade auspice to ${{ env.auspice_commit }}"
           author: 'nextstrain-bot <[email protected]>'
           committer: 'nextstrain-bot <[email protected]>'
-          title: '[bot] [DO NOT MERGE] Test auspice PR ${{ github.event.pull_request.number }}'
+          title: '[bot] [DO NOT MERGE] Test auspice ref ${{ github.event.workflow_dispatch.ref }}'
           body: |
-            This PR has been created to test Auspice from [PR ${{ github.event.pull_request.number }}](https://github.com/nextstrain/auspice/pull/${{ github.event.pull_request.number }})
+            This PR has been created to test Auspice from [ref ${{ github.event.workflow_dispatch.ref }}](https://github.com/nextstrain/auspice/pull/${{ github.event.workflow_dispatch.ref }})
 
             This message and corresponding commits were automatically created by a GitHub Action from [nextstrain/auspice](https://github.com/nextstrain/auspice)
           draft: true