scsynth - specify -U flag for plugin directory

Ensures only the Sonic Pi plugin directory is loaded. This isolates Sonic Pi’s scsynth from any externally installed SuperCollider instance which may have incompatible plugins.

Fixes boot errors caused by attempting to load plugins from an incompatible SuperCollider version.

Author: samaaron

app/server/sonicpi/lib/sonicpi/scsynthexternal.rb

@@ -342,15 +342,15 @@ def boot_server_osx
           raise "Unable to boot sound synthesis engine: the input and output rates of your audio card are not the same. Got in: #{audio_in_rate}, out: #{audio_out_rate}."
         end
       end
-      boot_and_wait(scsynth_path, "-u", @port.to_s, "-a", num_audio_busses_for_current_os.to_s, "-m", "131072", "-D", "0", "-R", "0", "-l", "1", "-i", "16", "-o", "16", "-b", num_buffers_for_current_os.to_s)
+      boot_and_wait(scsynth_path, "-u", @port.to_s, "-a", num_audio_busses_for_current_os.to_s, "-m", "131072", "-D", "0", "-R", "0", "-l", "1", "-i", "16", "-o", "16", "-b", num_buffers_for_current_os.to_s, "-U", "#{native_path}/supercollider/plugins/",)
     end
 
 
     def boot_server_windows
       log_boot_msg
       puts "Booting on Windows"
 
-      boot_and_wait(scsynth_path, "-u", @port.to_s, "-a", num_audio_busses_for_current_os.to_s, "-m", "131072", "-D", "0", "-R", "0", "-l", "1", "-i", "16", "-o", "16", "-b", num_buffers_for_current_os.to_s, "-B", "127.0.0.1")
+      boot_and_wait(scsynth_path, "-u", @port.to_s, "-a", num_audio_busses_for_current_os.to_s, "-m", "131072", "-D", "0", "-R", "0", "-l", "1", "-i", "16", "-o", "16", "-b", num_buffers_for_current_os.to_s, "-U", "#{native_path}/plugins/", "-B", "127.0.0.1")
     end
 
     def boot_server_raspberry_pi