Install .NET 6 since dotnet-validate seems to need that

ThomasArdal · ThomasArdal · commit f394d078e3bc · 2025-01-13T09:10:10.000+01:00
diff --git a/.github/workflows/build.yml b/.github/workflows/build.yml
@@ -18,6 +18,7 @@ jobs:
       uses: actions/setup-dotnet@v4
       with:
         dotnet-version: |
+          6.0.x
           8.0.x
           9.0.x
 
