windows: Use the 'WIX' environment variable to find heat.exe
windows: Use the 'WIX' environment variable to find heat.exe

On x64 systems, WIX installs itself in "Program Files (x86)" instead of
"Program Files," so the build-installer script cannot find `heat.exe`
in these cases. Using the `WIX` environment variable to locate the
binary solves this problem.

Signed-off-by: Gabriel Burt <ce11a5f747b579a775c140646376daaba8c3eb7d@gmail.com>

GNOME/banshee,dufoli/banshee,arfbtwn/banshee,mono-soc-2011/banshee,babycaseny/banshee,stsundermann/banshee,mono-soc-2011/banshee,arfbtwn/banshee,babycaseny/banshee,arfbtwn/banshee,babycaseny/banshee,arfbtwn/banshee,Carbenium/banshee,dufoli/banshee,stsundermann/banshee,dufoli/banshee,ixfalia/banshee,ixfalia/banshee,stsundermann/banshee,GNOME/banshee,dufoli/banshee,stsundermann/banshee,Dynalon/banshee-osx,arfbtwn/banshee,GNOME/banshee,ixfalia/banshee,arfbtwn/banshee,babycaseny/banshee,arfbtwn/banshee,mono-soc-2011/banshee,Carbenium/banshee,GNOME/banshee,GNOME/banshee,dufoli/banshee,ixfalia/banshee,Dynalon/banshee-osx,Dynalon/banshee-osx,Dynalon/banshee-osx,dufoli/banshee,GNOME/banshee,babycaseny/banshee,Dynalon/banshee-osx,stsundermann/banshee,Dynalon/banshee-osx,dufoli/banshee,ixfalia/banshee,Carbenium/banshee,Carbenium/banshee,GNOME/banshee,ixfalia/banshee,Dynalon/banshee-osx,babycaseny/banshee,stsundermann/banshee,Carbenium/banshee,mono-soc-2011/banshee,arfbtwn/banshee,babycaseny/banshee,ixfalia/banshee,GNOME/banshee,Dynalon/banshee-osx,babycaseny/banshee,mono-soc-2011/banshee,Carbenium/banshee,dufoli/banshee,stsundermann/banshee,mono-soc-2011/banshee,ixfalia/banshee,mono-soc-2011/banshee,stsundermann/banshee