[Infra] Ensure .NET 8 is installed (#6313)

Author: martincostello

.github/workflows/Component.BuildTest.yml

@@ -53,7 +53,13 @@ jobs:
         # retrieve them all.
         fetch-depth: 0
 
-    - name: Setup dotnet
+    - name: Setup previous .NET runtimes
+      uses: actions/setup-dotnet@67a3573c9a986a3f9c594539f4ab511d57bb3ce9 # v4.3.1
+      with:
+        dotnet-version: |
+          8.0.x
+
+    - name: Setup .NET
       uses: actions/setup-dotnet@67a3573c9a986a3f9c594539f4ab511d57bb3ce9 # v4.3.1
 
     - name: dotnet restore ${{ inputs.project-name }}