ci: bump actions/checkout from 2.3.5 to 2.4.0 (#668)

Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

.github/workflows/release.yml

@@ -9,7 +9,7 @@ jobs:
   release:
     runs-on: ubuntu-18.04
     steps:
-      - uses: actions/checkout@v2.3.5
+      - uses: actions/checkout@v2.4.0
       # https://github.com/peaceiris/workflows/blob/main/create-release-npm/action.yml
       - uses: peaceiris/workflows/create-release-npm@v0.11.0
         env: