[winswitch] Build-base override not working for build_conf

[Tim Vail]

Hey,

At least in Gentoo, the python does not copy the value of build_base
into all the commands.  It seems like the command that has a valid
build_base is the 'build' command.  I found that changing the
build_xpra_conf call from the build_conf command override to use the
build_base from the build command fixes it. Patch attached.
Alternatively, I suppose you could look into why build_base isn't
propagated to build_conf command.

Thanks,

-Tim