Add a dedicated "Manage PRs" workflow job for review request

The workflow result might indicate either that the PR author could require assistance from a maintainer or that something
is wrong with the system. In this case, the situation is brought to the attention of the maintainers by requesting a pull
request review from them.

Due to the need to avoid requesting review from a maintainer when they are the PR author (which is not allowed and thus
would result in a spurious workflow failure), the code for requesting this review is not as trivial as might be expected.
Previously, this code was duplicated at multiple places in the workflow, and would become more so as additional code is
added. The workflow is made cleaner by moving that duplicated code to a single dedicated job, which is facilitated by the
recent reworking of the workflow structure.

This is a pure refactoring and should have no effect on the workflow behavior.

.github/workflows/manage-prs.yml

@@ -470,37 +470,12 @@ jobs:
 
             Once that is done, it will be merged automatically.
 
-      - name: Request a review in case assistance is required
-        if: contains(toJSON(env.MAINTAINERS), github.actor) != true # Don't attempt to request review from PR author.
-        uses: octokit/request-action@v2.x
-        env:
-          GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
-        with:
-          route: POST /repos/{owner}/{repo}/pulls/{pull_number}/requested_reviewers
-          owner: ${{ github.repository_owner }}
-          repo: ${{ github.event.repository.name }}
-          pull_number: ${{ github.event.pull_request.number }}${{ github.event.issue.number }}
-          reviewers: ${{ env.MAINTAINERS }}
-
   not-submission:
     needs:
       - parse
     if: needs.parse.outputs.type != 'submission' # These request types can't be automatically approved.
     runs-on: ubuntu-latest
-
     steps:
-      - name: Request pull request review
-        if: contains(toJSON(env.MAINTAINERS), github.actor) != true
-        uses: octokit/request-action@v2.x
-        env:
-          GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
-        with:
-          route: POST /repos/{owner}/{repo}/pulls/{pull_number}/requested_reviewers
-          owner: ${{ github.repository_owner }}
-          repo: ${{ github.event.repository.name }}
-          pull_number: ${{ github.event.pull_request.number }}${{ github.event.issue.number }}
-          reviewers: ${{ env.MAINTAINERS }}
-
       - name: Comment on required review
         uses: octokit/request-action@v2.x
         env:
@@ -543,18 +518,6 @@ jobs:
           labels: |
             - "status: maintenance required"
 
-      - name: Request pull request review
-        if: contains(toJSON(env.MAINTAINERS), github.actor) != true
-        uses: octokit/request-action@v2.x
-        env:
-          GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
-        with:
-          route: POST /repos/{owner}/{repo}/pulls/{pull_number}/requested_reviewers
-          owner: ${{ github.repository_owner }}
-          repo: ${{ github.event.repository.name }}
-          pull_number: ${{ github.event.pull_request.number }}${{ github.event.issue.number }}
-          reviewers: ${{ env.MAINTAINERS }}
-
       - name: Comment on unexpected failure
         uses: octokit/request-action@v2.x
         env:
@@ -582,3 +545,29 @@ jobs:
             :warning::warning::warning::warning:
           SLACK_COLOR: danger
           MSG_MINIMAL: true
+
+  request-review:
+    needs:
+      - merge-fail
+      - not-submission
+      - unexpected-fail
+    if: >
+      always() &&
+      (
+        needs.merge-fail.result != 'skipped' ||
+        needs.not-submission.result != 'skipped' ||
+        needs.unexpected-fail.result != 'skipped'
+      )
+    runs-on: ubuntu-latest
+    steps:
+      - name: Request pull request review from maintainer
+        if: contains(toJSON(env.MAINTAINERS), github.actor) != true # Don't attempt to request review from PR author.
+        uses: octokit/request-action@v2.x
+        env:
+          GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
+        with:
+          route: POST /repos/{owner}/{repo}/pulls/{pull_number}/requested_reviewers
+          owner: ${{ github.repository_owner }}
+          repo: ${{ github.event.repository.name }}
+          pull_number: ${{ github.event.pull_request.number }}${{ github.event.issue.number }}
+          reviewers: ${{ env.MAINTAINERS }}