Bump actions/checkout from 2 to 2.3.4 (robotty#16)

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>

Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

Author: dependabot[bot]

.github/workflows/changelog-check.yml

@@ -9,7 +9,7 @@ jobs:
   check-changelog:
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@v2
+      - uses: actions/checkout@v2.3.4
 
       # Gives an error if there's no change in the changelog (except using label)
       - name: Changelog check

.github/workflows/rust.yml

@@ -17,7 +17,7 @@ jobs:
         os: [ubuntu-latest, windows-latest, macOS-latest]
         rust: [stable, nightly]
     steps:
-      - uses: actions/checkout@v2
+      - uses: actions/checkout@v2.3.4
       - uses: hecrj/setup-rust-action@v1
         with:
           rust-version: ${{ matrix.rust }}
@@ -29,7 +29,7 @@ jobs:
   check-formatting:
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@v2
+      - uses: actions/checkout@v2.3.4
       - uses: hecrj/setup-rust-action@v1
         with:
           rust-version: stable