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. With the manual trigger, there is an option to allow this workflow to
   run on tags and branches without PRs. To keep the logic simpler and
   maintain previous behavior, I've chosen to not allow those scenarios.
2. The GITHUB_SHA of workflow_dispatch is no longer the GitHub-managed
   PR merge ref, so it can be used here to retain original behavior.

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 +1,68 @@
 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 logic and wording easier.
+    if: github.ref_type == 'branch'
+
     runs-on: ubuntu-latest
+
+    env:
+      DESTINATION_REPO_DIR: nextstrain.org
+
     steps:
+      # Outputs:
+      # - pr-number: The PR number from the branch name (exits if no PR exists).
+      - name: Detect PR from branch
+        id: detect-pr
+        run: |
+          PR_NUMBER=$(gh pr view $GITHUB_REF_NAME --repo nextstrain/auspice --json 'number' --jq '.number') || true
+          if [[ -z $PR_NUMBER ]]; then
+            echo "ERROR: This branch is not associated with a PR in Auspice." >&2
+            exit 1
+          fi
+          echo "::set-output name=pr-number::$PR_NUMBER"
+        env:
+          GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
+
       - uses: actions/setup-node@v2
         with:
           node-version: '14'
+
       - name: Checkout nextstrain.org repo
         uses: actions/checkout@v2
         with:
           repository: nextstrain/nextstrain.org
+          path: ${{ env.DESTINATION_REPO_DIR }}
+
       - 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
+        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: 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:
+          path: ${{ env.DESTINATION_REPO_DIR }}
           token: ${{ secrets.NEXTSTRAIN_BOT_PAT }}
-          branch: "nextstrain-bot/test-auspice-pr/${{ github.event.pull_request.number }}"
-          commit-message: "[testing only] Upgrade Auspice to ${{ env.auspice_commit }}"
+          branch: "nextstrain-bot/test-auspice-pr/${{ steps.detect-pr.outputs.pr-number }}"
+          commit-message: "[testing only] Upgrade Auspice to ${{ github.sha }}"
           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 PR ${{ steps.detect-pr.outputs.pr-number }}'
           body: |
-            This PR has been created to test nextstrain.org running Auspice with changes from https://github.com/nextstrain/auspice/pull/${{ github.event.pull_request.number }}.
+            This PR has been created to test nextstrain.org running Auspice with changes from https://github.com/nextstrain/auspice/pull/${{ steps.detect-pr.outputs.pr-number }}.
 
             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 }}"