Try to reduce number of parallel AOT compilations

To check whether it will improve memory issues on CI

Author: radekdoulik

src/tasks/AotCompilerTask/MonoAOTCompiler.cs

@@ -523,7 +523,7 @@ private bool ExecuteInternal()
         }
         else
         {
-            int allowedParallelism = DisableParallelAot ? 1 : Math.Min(_assembliesToCompile.Count, Environment.ProcessorCount);
+            int allowedParallelism = DisableParallelAot ? 1 : Math.Min(_assembliesToCompile.Count, Math.Max(1, Environment.ProcessorCount/2));
             if (BuildEngine is IBuildEngine9 be9)
                 allowedParallelism = be9.RequestCores(allowedParallelism);