Skip .editorconfig discovery test for now

Our CI machines don't have a new enough MSBuild to support this; adding
it is tracked by https://github.com/dotnet/core-eng/issues/6424 which
is currently underway but there's no reason to hold up the larger
work on the test.

src/Workspaces/MSBuildTest/MSBuildWorkspaceTests.cs

@@ -3628,7 +3628,7 @@ public async Task TestDuplicateProjectReference()
             }
         }
 
-        [ConditionalFact(typeof(VisualStudioMSBuildInstalled)), Trait(Traits.Feature, Traits.Features.MSBuildWorkspace)]
+        [ConditionalFact(typeof(VisualStudioMSBuildInstalled), AlwaysSkip = "https://github.com/dotnet/core-eng/issues/6424"), Trait(Traits.Feature, Traits.Features.MSBuildWorkspace)]
         public async Task TestEditorConfigDiscovery()
         {
             var files = GetSimpleCSharpSolutionFiles()