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](https://github.com/actions/checkout/compare/v2...v2.3.4)

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

.github/workflows/coverage.yaml

@@ -15,7 +15,7 @@ jobs:
     timeout-minutes: 30
     runs-on: ubuntu-20.04
     steps:
-      - uses: actions/checkout@v2
+      - uses: actions/checkout@v2.3.4
 
       - uses: actions-rs/toolchain@v1
         with:

.github/workflows/main.yml

@@ -7,7 +7,7 @@ jobs:
     name: test on nightly
     runs-on: ubuntu-latest
     steps:
-    - uses: actions/checkout@master
+    - uses: actions/checkout@v2.3.4
     # Because we use nightly features for building docs,
     # using --all-features will fail without nightly toolchain.
     - uses: actions-rs/toolchain@v1