github: Update to newer checkout action

We're getting deprecation warnings about the v3 one.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>

.github/workflows/build.yml

@@ -22,7 +22,7 @@ jobs:
 
     steps:
       - name: Checkout
-        uses: actions/checkout@v3
+        uses: actions/checkout@v4
 
       - name: Install Dependencies
         run: |
@@ -49,7 +49,7 @@ jobs:
 
     steps:
       - name: Checkout
-        uses: actions/checkout@v3
+        uses: actions/checkout@v4
 
       - name: Install Dependencies
         run: |