pmb.build.init: fix apk wrapper for x86 on x86_64

Only install the abuild-apk wrapper if cpu emulation is required. This fixes building for x86 on x86_64.

Fix for:

>>> postmarketos-base-ui-gnome: Analyzing dependencies...
/usr/local/bin/abuild-apk: line 11: /native/usr/bin/abuild-apk: not found

Fixes: c5ca06d5 ("pmb: only enable abuild-apk wrapper for buildroot (MR 2246)")