Update actions/checkout requirement to v2.3.4

Updates the requirements on [actions/checkout](https://github.com/actions/checkout) to permit the latest version.
- [Release notes](https://github.com/actions/checkout/releases)
- [Changelog](https://github.com/actions/checkout/blob/main/CHANGELOG.md)
- [Commits](https://github.com/actions/checkout/commits/5a4ac9002d0be2fb38bd78e4b4dbde5606d7042f)

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

.github/workflows/format.yml

@@ -31,6 +31,6 @@ jobs:
           components: ${{ matrix.components || '' }}
           targets: ${{ matrix.targets || '' }}
       - name: Checkout
-        uses: actions/checkout@v1
+        uses: actions/checkout@v2.3.4
       - name: Check Formatting
         run: cargo fmt -- --check

.github/workflows/nightly.yml

@@ -42,7 +42,7 @@ jobs:
           components: ${{ matrix.components || '' }}
           targets: ${{ matrix.targets || '' }}
       - name: Checkout rusty-hermit
-        uses: actions/checkout@v2
+        uses: actions/checkout@v2.3.4
         with:
           repository: 'hermitcore/rusty-hermit'
           ref: 'master'
@@ -54,7 +54,7 @@ jobs:
         run: git rm -r libhermit-rs
         if: ${{ matrix.os == 'windows-latest' }}
       - name: Checkout
-        uses: actions/checkout@v2
+        uses: actions/checkout@v2.3.4
         with:
           path: 'libhermit-rs'
       - name: Check Cargo availability

.github/workflows/publish.yml

@@ -12,7 +12,7 @@ jobs:
 
     steps:
       - name: Checkout code
-        uses: actions/checkout@v1
+        uses: actions/checkout@v2.3.4
         with:
          submodules: true
       - name: Install nightly toolchain

.github/workflows/security_audit.yml

@@ -7,7 +7,7 @@ jobs:
   security_audit:
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@v1
+      - uses: actions/checkout@v2.3.4
       - uses: actions-rs/audit-check@v1
         with:
           token: ${{ secrets.GITHUB_TOKEN }}