Update actions/checkout action to v2.4.0

.github/workflows/rebase.yml

@@ -8,7 +8,7 @@ jobs:
     if: github.event.issue.pull_request != '' && contains(github.event.comment.body, '/rebase')
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@v2.3.5
+      - uses: actions/checkout@v2.4.0
         with:
           token: ${{ secrets.PAT_TOKEN }}
           fetch-depth: 0 # otherwise, you will failed to push refs to dest repo

.github/workflows/sync-release-version.yml

@@ -8,7 +8,7 @@ jobs:
   update-version:
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@v2.3.5
+      - uses: actions/checkout@v2.4.0
         with:
           fetch-depth: 0
       - name: Sync release version.

.github/workflows/update-readme.yml

@@ -9,7 +9,7 @@ jobs:
   sync-assets:
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@v2.3.5
+      - uses: actions/checkout@v2.4.0
         with:
           fetch-depth: 0