Improve sky_tools build so that it works from sky_engine
Improve sky_tools build so that it works from sky_engine

With this patch, we can switch over to using this command from the build system
in the sky_engine repo.

flutter/flutter,flutter/flutter,jason-simmons/flutter,tvolkert/flutter,jason-simmons/flutter,aghassemi/flutter,turnidge/flutter,tvolkert/flutter,turnidge/flutter,flutter/flutter,cbracken/flutter,Hixie/flutter,flutter/flutter,tvolkert/flutter,Hixie/flutter,jason-simmons/flutter,Hixie/flutter,cbracken/flutter,Hixie/flutter,flutter/flutter,tvolkert/flutter,tvolkert/flutter,Hixie/flutter,turnidge/flutter,cbracken/flutter,turnidge/flutter,flutter/flutter,tvolkert/flutter,cbracken/flutter,tvolkert/flutter,aghassemi/flutter,aghassemi/flutter,cbracken/flutter,Hixie/flutter,jason-simmons/flutter,Hixie/flutter,tvolkert/flutter,cbracken/flutter,cbracken/flutter,turnidge/flutter,flutter/flutter,aghassemi/flutter,cbracken/flutter,cbracken/flutter,Hixie/flutter,cbracken/flutter,flutter/flutter,tvolkert/flutter,tvolkert/flutter,flutter/flutter,flutter/flutter,Hixie/flutter,Hixie/flutter