GitHub CI: Switch actions/checkout from v4 to v6

Note 1: checkout/v4 runs with deprecated Node.js 20, whereas
  checkout/v6 uses current Node.js 24 (recommended by GitHub).

Note 2: GitHub will remove Node.js 20 in the near future (fall 2026).

.github/workflows/build.yml

@@ -18,7 +18,7 @@ jobs:
     runs-on: ubuntu-latest
 
     steps:
-    - uses: actions/checkout@v4
+    - uses: actions/checkout@v6
 
     - name: install prerequisites
       run: sudo apt-get update -y && sudo apt-get install -y libasound2-dev libglew-dev libglu1-mesa-dev libcairo2-dev libx11-dev libxcursor-dev libxft-dev libxinerama-dev
@@ -54,7 +54,7 @@ jobs:
     runs-on: ubuntu-latest
 
     steps:
-    - uses: actions/checkout@v4
+    - uses: actions/checkout@v6
 
     - name: Install prerequisites
       run: sudo apt-get update -y && sudo apt-get install -y libasound2-dev libglu1-mesa-dev libcairo2-dev libwayland-dev wayland-protocols libdbus-1-dev libxkbcommon-dev libegl-dev libopengl-dev libpango1.0-dev libdecor-0-dev libxfixes-dev libxcursor-dev libxinerama-dev
@@ -88,7 +88,7 @@ jobs:
     runs-on: macos-latest
 
     steps:
-    - uses: actions/checkout@v4
+    - uses: actions/checkout@v6
 
     - name: Create Build Environment
       # Some projects don't allow in-source building, so create a separate build directory
@@ -123,7 +123,7 @@ jobs:
     runs-on: windows-latest
 
     steps:
-    - uses: actions/checkout@v4
+    - uses: actions/checkout@v6
 
     - name: Create Build Environment
       run: cmake -E make_directory ${{github.workspace}}/build