Bump actions/checkout from 2 to 4

Bumps [actions/checkout](https://github.com/actions/checkout) from 2 to 4.
- [Release notes](https://github.com/actions/checkout/releases)
- [Changelog](https://github.com/actions/checkout/blob/main/CHANGELOG.md)
- [Commits](actions/checkout@v2...v4)

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

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

.github/workflows/ci.yml

@@ -88,7 +88,7 @@ jobs:
 
     - run: df -h .
 
-    - uses: actions/checkout@v3.1.0
+    - uses: actions/checkout@v4
 
     - run: df -h .
 
@@ -240,7 +240,7 @@ jobs:
         large-packages: false # slow
         swap-storage: false
 
-    - uses: actions/checkout@v3.1.0
+    - uses: actions/checkout@v4
 
     - name: Install native libraries
       if: ${{ runner.os == 'Linux' }}
@@ -299,7 +299,7 @@ jobs:
     runs-on: ubuntu-latest
 
     steps:
-    - uses: actions/checkout@v3.1.0
+    - uses: actions/checkout@v4
 
     - uses: Swatinem/rust-cache@v2.7.0
 
@@ -321,7 +321,7 @@ jobs:
     runs-on: ubuntu-latest
 
     steps:
-    - uses: actions/checkout@v3.1.0
+    - uses: actions/checkout@v4
 
     # The build is faster *without* this cache.
     # - uses: Swatinem/rust-cache@v2.7.0
@@ -345,7 +345,7 @@ jobs:
     runs-on: ubuntu-latest
 
     steps:
-    - uses: actions/checkout@v3.1.0
+    - uses: actions/checkout@v4
 
     # Cache both the main workspace (for xtask builds) and the fuzzing workspace
     - uses: Swatinem/rust-cache@v2.7.0

.github/workflows/mutants.yml

@@ -12,7 +12,7 @@ jobs:
     runs-on: ubuntu-latest
 
     steps:
-      - uses: actions/checkout@v2
+      - uses: actions/checkout@v4
 
       - name: Install Rust toolchain
         # Not sure if this is strictly necessary -- but it is if GitHub Actions images aren't

.github/workflows/unstable-ci.yml

@@ -59,7 +59,7 @@ jobs:
     runs-on: ${{ matrix.os }}-latest
 
     steps:
-    - uses: actions/checkout@v3.1.0
+    - uses: actions/checkout@v4
       with:
         ref: unstable-rust
 
@@ -139,7 +139,7 @@ jobs:
     runs-on: ubuntu-latest
 
     steps:
-    - uses: actions/checkout@v3.1.0
+    - uses: actions/checkout@v4
 
     - uses: Swatinem/rust-cache@v2.7.0
       with: