Don't use $PKG_CONFIG for wayland-scanner

I made a mistake in the previous commit — $PKG_CONFIG should not be used to find wayland-scanner, because when cross-compiling that'll look for the cross-compiled wayland-scanner. I missed this because I tested by compiling from glibc to musl on the same architecture, so I was testing a special case where my system could actually execute the cross-compiled wayland-scanner.