Bump checkout action to v4

This should have no effect for us since we only use default features.

Author: mgeisler

.github/workflows/test.yml

@@ -16,7 +16,7 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - name: Checkout repository
-        uses: actions/checkout@v3
+        uses: actions/checkout@v4
 
       - uses: Swatinem/rust-cache@v2
         with:
@@ -30,7 +30,7 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - name: Checkout repository
-        uses: actions/checkout@v3
+        uses: actions/checkout@v4
 
       - name: Install nightly Rust
         run: rustup default nightly
@@ -72,7 +72,7 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - name: Checkout repository
-        uses: actions/checkout@v3
+        uses: actions/checkout@v4
 
       - uses: Swatinem/rust-cache@v2
 
@@ -88,7 +88,7 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - name: Checkout repository
-        uses: actions/checkout@v3
+        uses: actions/checkout@v4
 
       # We use an unstable rustmft feature and we thus need the
       # nightly channel to enforce the formatting.