CI: Use checkout@v3

Also restrict ourselves to shallow clones for the Build workflow.

.github/workflows/build.yml

@@ -27,9 +27,10 @@ jobs:
       shell: bash
 
 
-    - uses: actions/checkout@v2
+    - uses: actions/checkout@v3
       with:
           submodules: 'recursive'
+          fetch-depth: 1
 
 
     - name: Set environment variables

.github/workflows/pr-checks.yml

@@ -12,7 +12,7 @@ jobs:
       run: sudo apt install python3
       shell: bash
 
-    - uses: actions/checkout@v2
+    - uses: actions/checkout@v3
       with:
           # Assume that there are not >200 new commits that need checking
           fetch-depth: 200