Bump actions/checkout from 3 to 4

Bumps [actions/checkout](https://github.com/actions/checkout) from 3 to 4.
- [Release notes](https://github.com/actions/checkout/releases)
- [Changelog](https://github.com/actions/checkout/blob/main/CHANGELOG.md)
- [Commits](actions/checkout@v3...v4)

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

Signed-off-by: dependabot[bot] <[email protected]>

.github/workflows/ci.yml

@@ -12,7 +12,7 @@ jobs:
  #- { rust: stable, required: false }
  - { rust: nightly, required: true }
  steps:
- - uses: actions/checkout@v3
+ - uses: actions/checkout@v4
  - uses: dtolnay/rust-toolchain@master
  with:
  toolchain: ${{ matrix.rust }}
@@ -34,7 +34,7 @@ jobs:
  runs-on: ubuntu-latest
  if: ${{ github.actor != 'dependabot[bot]' }}
  steps:
- - uses: actions/checkout@v3
+ - uses: actions/checkout@v4
  - uses: dtolnay/rust-toolchain@master
  with:
  toolchain: nightly