workflows: Unsplit pr-subscriber (llvm#69748)

This is essentially a revert of
1ed7108. It is safe to use the
pull_request_target event for pr-subscriber, because it does not
checkout any code from the pull request branch.

Author: tstellar

.github/workflows/pr-receive-label.yml

@@ -1,22 +1,17 @@
 name: PR Subscriber
 
 on:
-  workflow_run:
-    workflows: ["PR Receive Label"]
+  pull_request_target:
     types:
-      - completed
+      - labeled
 
 permissions:
-  actions: read
   contents: read
 
 jobs:
   auto-subscribe:
     runs-on: ubuntu-latest
-    if: >
-      github.repository == 'llvm/llvm-project' &&
-      github.event.workflow_run.event == 'pull_request' &&
-      github.event.workflow_run.conclusion == 'success'
+    if: github.repository == 'llvm/llvm-project'
     steps:
       - name: Setup Automation Script
         run: |
@@ -26,47 +21,10 @@ jobs:
           chmod a+x github-automation.py
           pip install -r requirements.txt
 
-      - name: 'Wait for other actions'
-        # We can't use the concurrency tag for these jobs, because it will
-        # cancel pending jobs if another job is running.
-        env:
-          GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
-        run: |
-          python3 pr-subscriber-wait.py
-
-
-      # From: https://securitylab.github.com/research/github-actions-preventing-pwn-requests/
-      # Updated version here: https://docs.github.com/en/actions/using-workflows/events-that-trigger-workflows#using-data-from-the-triggering-workflow
-      - name: 'Download artifact'
-        uses: actions/github-script@v6
-        with:
-          script: |
-            const artifacts = await github.rest.actions.listWorkflowRunArtifacts({
-               owner: context.repo.owner,
-               repo: context.repo.repo,
-               run_id: context.payload.workflow_run.id
-            });
-            const matchArtifact = artifacts.data.artifacts.find((artifact) =>
-              artifact.name === 'pr'
-            );
-            const download = await github.rest.actions.downloadArtifact({
-               owner: context.repo.owner,
-               repo: context.repo.repo,
-               artifact_id: matchArtifact.id,
-               archive_format: 'zip'
-            });
-            const { writeFileSync } = require('node:fs');
-            writeFileSync('${{ github.workspace }}/pr.zip', Buffer.from(download.data));
-
-      - run: unzip pr.zip
-
       - name: Update watchers
-        # https://docs.github.com/en/actions/security-guides/security-hardening-for-github-actions#using-an-intermediate-environment-variable
         run: |
-          PR_NUMBER=$(cat NR)
-          LABEL_NAME=$(cat LABEL)
           ./github-automation.py \
             --token '${{ secrets.ISSUE_SUBSCRIBER_TOKEN }}' \
             pr-subscriber \
-            --issue-number "$PR_NUMBER" \
-            --label-name "$LABEL_NAME"
+            --issue-number "${{ github.event.number }}" \
+            --label-name "${{ github.event.label.name }}"