Work around long socket path on tvOS.

tvOS has a MAX_PATH of 104 characters on domain sockets, and a 90 character
value for GetTempPath. That gives us only 14 characters to play with, and
temp filenames are 12 characters long. We cannot afford the `CoreFxPipe_`
prefix on tvOS, with our tiny character path budget.

Check whether this fixes some or all of dotnet#67853 and dotnet#51390 too.

src/libraries/System.IO.Pipes/src/System/IO/Pipes/PipeStream.Unix.cs

@@ -30,7 +30,7 @@ public abstract partial class PipeStream : Stream
         private static readonly char[] s_invalidPathNameChars = Path.GetInvalidPathChars();
 
         /// <summary>Prefix to prepend to all pipe names.</summary>
-        private static readonly string s_pipePrefix = Path.Combine(Path.GetTempPath(), "CoreFxPipe_");
+        private static readonly string s_pipePrefix = Path.Combine(Path.GetTempPath(), (RuntimeInformation.IsOSPlatform(OSPlatform.Create("TVOS"))) ? "" : "CoreFxPipe_");
 
         public override int Read(byte[] buffer, int offset, int count)
         {