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

@@ -31,7 +31,7 @@ jobs:
     name: Build (Go ${{ matrix.go }})
     runs-on: ubuntu-latest
     steps:
-    - uses: actions/checkout@v3
+    - uses: actions/checkout@v4
     - uses: actions/setup-go@v2
       with:
         go-version: ${{ matrix.go }}
@@ -46,7 +46,7 @@ jobs:
     name: Build (32-bit)
     runs-on: ubuntu-latest
     steps:
-    - uses: actions/checkout@v3
+    - uses: actions/checkout@v4
     - uses: actions/setup-go@v2
       with:
         go-version: '1.20'
@@ -62,7 +62,7 @@ jobs:
     name: Run integration tests
     runs-on: ubuntu-latest
     steps:
-    - uses: actions/checkout@v3
+    - uses: actions/checkout@v4
     - uses: actions/setup-go@v2
       with:
         go-version: '1.20'
@@ -88,7 +88,7 @@ jobs:
   #       arch: [armv7, aarch64, ppc64le]
   #   runs-on: ubuntu-latest
   #   steps:
-  #     - uses: actions/checkout@v3
+  #     - uses: actions/checkout@v4
   #     - uses: uraimo/run-on-arch-action@v2.0.5
   #       with:
   #         arch: ${{ matrix.arch }}
@@ -111,7 +111,7 @@ jobs:
     # The cli tests require kernel 5.4 or later, and thus Ubuntu 20.04 or later.
     runs-on: ubuntu-20.04
     steps:
-    - uses: actions/checkout@v3
+    - uses: actions/checkout@v4
     - uses: actions/setup-go@v2
       with:
         go-version: '1.20'
@@ -126,7 +126,7 @@ jobs:
     name: Generate, format, and lint
     runs-on: ubuntu-latest
     steps:
-    - uses: actions/checkout@v3
+    - uses: actions/checkout@v4
     - uses: actions/setup-go@v2
       with:
         go-version: '1.20'