ci: update workflows for breaking actions/checkout change

Problem: actions/checkout@v7 refuses to checkout fork pull request code
on `pull_request_target` trigger, breaking backport and reviewers-remove
workflows on external PRs.

Solution:
* backport: Checkout the merged-into base branch by name.
* reviewers-remove: inline script so no checkout needed.

.github/scripts/reviewers_remove.js

@@ -1,16 +0,0 @@
-module.exports = async ({ github, context }) => {
-  const requestedReviewers = await github.rest.pulls.listRequestedReviewers({
-    owner: context.repo.owner,
-    repo: context.repo.repo,
-    pull_number: context.issue.number,
-  });
-
-  const reviewers = requestedReviewers.data.users.map((e) => e.login);
-
-  github.rest.pulls.removeRequestedReviewers({
-    owner: context.repo.owner,
-    repo: context.repo.repo,
-    pull_number: context.issue.number,
-    reviewers: reviewers,
-  });
-};

.github/workflows/backport.yml

@@ -15,6 +15,9 @@ jobs:
     steps:
       - name: Checkout repository
         uses: actions/checkout@v7.0.0
+        with:
+          persist-credentials: true
+          ref: ${{ github.event.pull_request.base.ref }}
 
       - name: Create app token
         uses: actions/create-github-app-token@v3

.github/workflows/reviewers_remove.yml

@@ -11,15 +11,21 @@ jobs:
   remove-reviewers:
     runs-on: ubuntu-latest
     steps:
-      - name: Checkout script
-        uses: actions/checkout@v7.0.0
-        with:
-          sparse-checkout: .github/scripts/reviewers_remove.js
-          sparse-checkout-cone-mode: false
-
-      - name: Run script
+      - name: Remove reviewers
         uses: actions/github-script@v9
         with:
           script: |
-            const script = require('./.github/scripts/reviewers_remove.js')
-            await script({github, context})
+            const requestedReviewers = await github.rest.pulls.listRequestedReviewers({
+              owner: context.repo.owner,
+              repo: context.repo.repo,
+              pull_number: context.issue.number,
+            });
+
+            const reviewers = requestedReviewers.data.users.map((e) => e.login);
+
+            github.rest.pulls.removeRequestedReviewers({
+              owner: context.repo.owner,
+              repo: context.repo.repo,
+              pull_number: context.issue.number,
+              reviewers: reviewers,
+            });