ci.yml: use actions/checkout@v4

This addresses the following warning from GitHub Actions:

    Node.js 16 actions are deprecated. Please update the following
    actions to use Node.js 20: actions/checkout@v3. For more information
    see:
    https://github.blog/changelog/2023-09-22-github-actions-transitioning-from-node-16-to-node-20/.

A near-identical warning forced everyone to upgrade to v3 last year, so
this is some pointless churn, but let's just get it over with again...

.github/workflows/ci.yml

@@ -34,7 +34,7 @@ jobs:
         compiler: [gcc, clang]
     runs-on: ubuntu-latest
     steps:
-    - uses: actions/checkout@v3
+    - uses: actions/checkout@v4
     - name: Install dependencies
       run: |
         sudo apt-get update
@@ -50,7 +50,7 @@ jobs:
     name: Build and test (32-bit)
     runs-on: ubuntu-latest
     steps:
-    - uses: actions/checkout@v3
+    - uses: actions/checkout@v4
     - name: Install dependencies
       run: |
         sudo apt-get update
@@ -62,7 +62,7 @@ jobs:
     name: Build and test (valgrind enabled)
     runs-on: ubuntu-latest
     steps:
-    - uses: actions/checkout@v3
+    - uses: actions/checkout@v4
     - name: Install dependencies
       run: |
         sudo apt-get update
@@ -74,7 +74,7 @@ jobs:
     name: Build and test (UBSAN enabled)
     runs-on: ubuntu-latest
     steps:
-    - uses: actions/checkout@v3
+    - uses: actions/checkout@v4
     - name: Install dependencies
       run: |
         sudo apt-get update
@@ -86,7 +86,7 @@ jobs:
     name: Build and test (ASAN enabled)
     runs-on: ubuntu-latest
     steps:
-    - uses: actions/checkout@v3
+    - uses: actions/checkout@v4
     - name: Install dependencies
       run: |
         sudo apt-get update
@@ -98,7 +98,7 @@ jobs:
     name: Check source code formatting
     runs-on: ubuntu-latest
     steps:
-    - uses: actions/checkout@v3
+    - uses: actions/checkout@v4
     - name: Install dependencies
       run: |
         sudo apt-get update
@@ -110,7 +110,7 @@ jobs:
     name: Run clang static analyzer
     runs-on: ubuntu-latest
     steps:
-    - uses: actions/checkout@v3
+    - uses: actions/checkout@v4
     - name: Install dependencies
       run: |
         sudo apt-get update