GitHub Actions: switch to setup-go@v4 which enables caching

.github/workflows/push.yml

@@ -13,15 +13,15 @@ jobs:
     runs-on: ubuntu-latest
     steps:
 
+    - uses: actions/checkout@v3
+
     - name: Set up Go 1.x
-      uses: actions/setup-go@v2
+      uses: actions/setup-go@v4
       with:
-        # Run on the latest minor release of Go 1.19:
-        go-version: ^1.19
+        # Run on the latest minor release of Go 1.20:
+        go-version: ^1.20
       id: go
 
-    - name: Check out code into the Go module directory
-      uses: actions/checkout@v2
 
     - name: Ensure all files were formatted as per gofmt
       run: |