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.

Two notes:

1. 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.
2. The commit to install Auspice from is changing from the
   GitHub-managed PR merge HEAD to the source ref HEAD. This can have
   implications when run on a PR's source branch if there are notable
   changes in the target branch unincorporated in the source branch. To
   mitigate this, make sure to update the source branch (e.g. with a git
   rebase) before triggering this workflow.

.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
@@ -12,30 +12,24 @@ jobs:
         uses: actions/checkout@v2
         with:
           repository: nextstrain/nextstrain.org
-      - name: Install Auspice from PRs HEAD commit
-        if: ${{ github.event_name == 'pull_request' }}
-        # Note: $GITHUB_SHA is _not_ the same commit as the HEAD commit on the PR branch
-        # see https://gitpro.ttaallkk.topmunity/t/github-sha-not-the-same-as-the-triggering-commit/18286/2
+      - name: Install Auspice from source ref HEAD
         shell: bash
         run: |
-          AUSPICE_COMMIT=$(cat $GITHUB_EVENT_PATH | jq -r .pull_request.head.sha)
-          echo "auspice_commit=$AUSPICE_COMMIT" >> $GITHUB_ENV
           npm ci
-          npm install nextstrain/auspice#${AUSPICE_COMMIT}
+          npm install nextstrain/auspice#${GITHUB_SHA}
           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 }}"
-          commit-message: "[testing only] upgrade auspice to ${{ env.auspice_commit }}"
+          branch: "auspice-pr-${{ github.event.workflow_dispatch.ref }}"
+          commit-message: "[testing only] upgrade auspice to ${{ github.sha }}"
           author: 'nextstrain-bot <nextstrain-bot@users.noreply.github.com>'
           committer: 'nextstrain-bot <nextstrain-bot@users.noreply.github.com>'
-          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