Pin .NET 8.0.403

Author: jamescrosswell

.github/actions/environment/action.yml

@@ -55,7 +55,7 @@ runs:
         dotnet-version: |
           6.0.x
           7.0.x
-          8.0.x
+          8.0.403
 
     - name: Install .NET Workloads
       shell: bash