Check for Java in local SignalR build (#10455)

src/SignalR/build.cmd

@@ -1,3 +1,3 @@
 @ECHO OFF
 SET RepoRoot=%~dp0..\..\
-%RepoRoot%build.cmd -projects %~dp0**\*.*proj %*
+%RepoRoot%build.cmd -buildJava -projects %~dp0**\*.*proj %*