Bump actions/checkout from 2.3.5 to 2.4.0

Bumps [actions/checkout](https://github.com/actions/checkout) from 2.3.5 to 2.4.0.
- [Release notes](https://github.com/actions/checkout/releases)
- [Changelog](https://github.com/actions/checkout/blob/main/CHANGELOG.md)
- [Commits](https://github.com/actions/checkout/compare/v2.3.5...v2.4.0)

---
updated-dependencies:
- dependency-name: actions/checkout
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

Signed-off-by: dependabot[bot] <support@github.com>

.github/workflows/main.yml

@@ -18,7 +18,7 @@ jobs:
         os: [ubuntu-latest, windows-latest, macOS-latest]
 
     steps:
-      - uses: actions/checkout@v2.3.5
+      - uses: actions/checkout@v2.4.0
       - uses: actions-rs/toolchain@v1.0.7
         with:
           toolchain: nightly
@@ -43,7 +43,7 @@ jobs:
     name: Clippy (+nightly)
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@v2.3.5
+      - uses: actions/checkout@v2.4.0
       - uses: actions-rs/toolchain@v1.0.7
         with:
           toolchain: nightly
@@ -59,7 +59,7 @@ jobs:
     name: Code Coverage (+nightly)
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@v2.3.5
+      - uses: actions/checkout@v2.4.0
       - uses: actions-rs/toolchain@v1.0.7
         with:
           toolchain: nightly