chore(deps): Bump actions/checkout in /.github/actions/prepare-source

Bumps [actions/checkout](https://github.com/actions/checkout) from 4.1.7 to 4.2.0.
- [Release notes](https://github.com/actions/checkout/releases)
- [Changelog](https://github.com/actions/checkout/blob/main/CHANGELOG.md)
- [Commits](actions/checkout@v4.1.7...v4.2.0)

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

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

.github/actions/prepare-source/action.yml

@@ -4,7 +4,7 @@ runs:
  using: composite
  steps:
  - name: Check out mu-plugins-ext
- uses: actions/checkout@v4.1.7
+ uses: actions/checkout@v4.2.0
  with:
  repository: 'Automattic/vip-go-mu-plugins-ext'
  path: 'vip-go-mu-plugins-ext'