Fix workflows

- Install the .NET 8 SDK for testing.
- Install the .NET SDK from `global.json` before running `dotnet format`.

Author: martincostello

.github/workflows/ci.yml

@@ -53,6 +53,11 @@ jobs:
         filter: 'tree:0'
         show-progress: false
 
+    - name: Setup .NET 8 SDK
+      uses: actions/setup-dotnet@67a3573c9a986a3f9c594539f4ab511d57bb3ce9 # v4.3.1
+      with:
+        dotnet-version: '8.0.x'
+
     - name: Setup .NET SDK
       uses: actions/setup-dotnet@d4c94342e560b34958eacfc5d055d21461ed1c5d # v5.0.0
       id: setup-dotnet