Fix bug where debugger.mojo not found in some build configurations

BUG=
R=ppi@chromium.org

Review URL: https://codereview.chromium.org/2070303002 .

Bug: https://github.com/domokit/mojo/issues/787
Cr-Mirrored-From: https://github.com/domokit/mojo
Cr-Mirrored-Commit: 22a1435992c69b0d6d067c7ce3e91f5d54ad132f

mojo_run

diff --git a/mojo_run b/mojo_run
index 72fc047..fe85ea5 100755
--- a/mojo_run
+++ b/mojo_run
@@ -33,7 +33,7 @@
 _MOJO_DEBUGGER_PORT = 7777
 _LAUNCHER = 'mojo:launcher'
 
-_DEBUGGER_URL = 'https://core.mojoapps.io/debugger.mojo'
+_DEBUGGER_URL = 'mojo:debugger'
 
 
 def _configure_debugger(shell):