React to .NET 5 branding changes (#1399)

* React to .NET 5 branding changes

Author: jkotas

src/BenchmarkDotNet/Portability/RuntimeInformation.cs

@@ -31,13 +31,15 @@ internal static class RuntimeInformation
         [PublicAPI]
         public static bool IsNetNative => FrameworkDescription.StartsWith(".NET Native", StringComparison.OrdinalIgnoreCase);
 
-        public static bool IsNetCore => FrameworkDescription.StartsWith(".NET Core", StringComparison.OrdinalIgnoreCase) && !string.IsNullOrEmpty(typeof(object).Assembly.Location);
+        public static bool IsNetCore
+            => ((Environment.Version.Major >= 5) || FrameworkDescription.StartsWith(".NET Core", StringComparison.OrdinalIgnoreCase))
+                && !string.IsNullOrEmpty(typeof(object).Assembly.Location);
 
         /// <summary>
         /// "The north star for CoreRT is to be a flavor of .NET Core" -> CoreRT reports .NET Core everywhere
         /// </summary>
         public static bool IsCoreRT
-            => FrameworkDescription.StartsWith(".NET Core", StringComparison.OrdinalIgnoreCase)
+            => ((Environment.Version.Major >= 5) || FrameworkDescription.StartsWith(".NET Core", StringComparison.OrdinalIgnoreCase))
                && string.IsNullOrEmpty(typeof(object).Assembly.Location); // but it's merged to a single .exe and .Location returns null here ;)
 
         public static bool IsRunningInContainer => string.Equals(Environment.GetEnvironmentVariable("DOTNET_RUNNING_IN_CONTAINER"), "true");