makechrootpkg: Reset environment for builduser

The gnustep-base package ships a profile.d script that adds
"$HOME/GNUstep/Tools" to the PATH, which breaks when the user changes
and causes meson to exit with a "permission denied" error.

makechrootpkg.in

@@ -199,9 +199,7 @@ EOF
 # so no global variables
 _chrootbuild() {
 	. /etc/profile
-	export HOME=/build
-	cd /startdir
-	sudo -u builduser makepkg "$@"
+	sudo -iu builduser bash -c 'cd /startdir; makepkg "$@"' -bash "$@"
 }
 
 _chrootnamcap() {