Bump actions/checkout from 2 to 2.3.4

Bumps [actions/checkout](https://github.com/actions/checkout) from 2 to 2.3.4. - [Release notes](https://github.com/actions/checkout/releases) - [Changelog](https://github.com/actions/checkout/blob/main/CHANGELOG.md) - [Commits](actions/checkout@v2...v2.3.4) Signed-off-by: dependabot[bot] <support@github.com>

Author: dependabot[bot]

.github/workflows/cargo-test.yml

@@ -8,7 +8,7 @@ jobs:
  runs-on: ubuntu-latest
  steps:
  - name: Checkout sources
- uses: actions/checkout@v2
+ uses: actions/checkout@v2.3.4
  with:
  submodules: true