[4.3] Install older .NET 6 on Linux CI to fix compiling

.github/workflows/linux_builds.yml

@@ -153,7 +153,9 @@ jobs:
         uses: actions/setup-dotnet@v4
         with:
           # Targeting the oldest version we want to support to ensure it still builds.
-          dotnet-version: 8.0.100
+          dotnet-version: |
+              6.0.100
+              8.0.100
 
       - name: Compilation
         uses: ./.github/actions/godot-build