Use Environment.SystemPageSize to set desireBufferSize (dotnet#40247)

PAGE_SIZE is not all 4096 for all architecture, so we can get it by Environment.SystemPageSize

Co-authored-by: Sunguoyun <[email protected]>

src/libraries/System.IO.Pipes/tests/AnonymousPipeTests/AnonymousPipeTest.Specific.cs

@@ -85,7 +85,7 @@ public static void Linux_BufferSizeRoundtrips()
             // On Linux, setting the buffer size of the server will also set the buffer size of the
             // client, regardless of the direction of the flow
 
-            int desiredBufferSize = 4096;
+            int desiredBufferSize = Environment.SystemPageSize;
             using (var server = new AnonymousPipeServerStream(PipeDirection.Out, HandleInheritability.None, desiredBufferSize))
             {
                 Assert.Equal(desiredBufferSize, server.OutBufferSize);