Hardcode executable path so that the symlink in the standard PATH
works correctly.

Index: jruby-launcher/jrubyexe.cpp
--- jruby-launcher/jrubyexe.cpp.orig
+++ jruby-launcher/jrubyexe.cpp
@@ -57,8 +57,10 @@ const char *CON_ATTACH_MSG =
 #include "unixlauncher.h"
 #endif  // WIN32
 
+char JRUBY_PATH[] = "${PREFIX}/jruby/bin/jruby";
 
 int main(int argc, char *argv[], char* envp[]) {
+    argv[0] = JRUBY_PATH;
 #ifdef WIN32
     checkLoggingArg(argc, argv, true);