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 of the generated PR has been updated to use the source
   branch name rather than PR number, since that is not guaranteed
   with the manual trigger. For the PR description, the workflow will
   link to either the PR or branch if no PR exists.
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.

Also, do all the nextstrain.org work under a separate directory
DESTINATION_REPO_DIR to make steps a bit more distinct.

.github/workflows/make_prs_for_other_repos.yaml

@@ -1,45 +1,66 @@
 name: "Make PRs for Nextstrain projects which depend on Auspice"
 on:
-  pull_request:
+  workflow_dispatch:
 jobs:
   make-pr-on-nextstrain-dot-org: # <job_id>
+    # I don't see this being used for tags, so ensure it's only run on branches
+    # to make subsequent wording easier.
+    if: github.ref_type == 'branch'
+
     runs-on: ubuntu-latest
+
+    env:
+      DESTINATION_REPO_DIR: nextstrain.org
+
     steps:
       - uses: actions/setup-node@v2
         with:
           node-version: '14'
+
       - name: Checkout nextstrain.org repo
         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
+          path: ${{ env.DESTINATION_REPO_DIR }}
+
+      - name: Install Auspice from source branch HEAD commit
         shell: bash
+        working-directory: ${{ env.DESTINATION_REPO_DIR }}
         run: |
-          AUSPICE_COMMIT=$(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#${GITHUB_SHA}
           git add package.json package-lock.json
+
+      - name: Generate Markdown link
+        id: generate-markdown-link
+        run: |
+          PR_NUMBER=$(gh pr view $GITHUB_REF_NAME --repo nextstrain/auspice --json 'number' --jq '.number') || true
+          if [[ -n $PR_NUMBER ]]; then
+            echo "::set-output name=text::[PR $PR_NUMBER](https://github.com/nextstrain/auspice/pull/$PR_NUMBER)"
+          else
+            echo "::set-output name=text::[branch $GITHUB_REF_NAME](https://github.com/nextstrain/auspice/tree/$GITHUB_REF_NAME)"
+          fi
+        env:
+          GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
+
       - 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
+        path: ${{ env.DESTINATION_REPO_DIR }}
         with:
           token: ${{ secrets.NEXTSTRAIN_BOT_PAT }}
-          branch: "nextstrain-bot/test-auspice-pr/${{ github.event.pull_request.number }}"
-          commit-message: "[testing only] Upgrade Auspice to ${{ env.auspice_commit }}"
+          branch: "nextstrain-bot/test-auspice-branch/${{ github.ref_name }}"
+          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 branch ${{ github.ref_name }}'
           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 ${{ steps.generate-markdown-link.outputs.text }}.
 
             This message and corresponding commits were automatically created by a GitHub Action from [nextstrain/auspice](https://github.com/nextstrain/auspice).
           draft: true
           delete-branch: true
+
       - name: Check outputs
         run: |
           echo "Nextstrain.org PR: ${{ steps.cpr.outputs.pull-request-number }}"