scripts/make: ignore unitTests warns if legacy

Somehow, unit tests started to throw strings to StdErr (like
'WARNING: The runtime version supported by this application
is unavailable.') while using dotnet6 it doesn't happen so
we ignore the warnings only if running in legacy mode.

This should fix CI after the last merge.

scripts/make.fsx

@@ -433,8 +433,17 @@ match maybeTarget with
             }
 #endif
 
-    Process.Execute(runnerCommand, Echo.All).UnwrapDefault()
+    let procResult = Process.Execute(runnerCommand, Echo.All)
+#if !LEGACY_FRAMEWORK
+    procResult.UnwrapDefault()
     |> ignore<string>
+#else // in legacy mode, warnings (output to StdErr) happen even if exitCode=0
+    match procResult.Result with
+    | ProcessResultState.Error (_, _) ->
+        failwith "Unit tests failed ^"
+    | _ ->
+        ()
+#endif
 
 | Some("install") ->
     let buildConfig = BinaryConfig.Release