Do not assume the makechrootpkg user's groupname is the same as the username

chown support "$user:$group" but also "$user:" which infers $group
rather than leaving it as root. This looks up the group name in cases
where the default group is e.g. "users" and users do not get their own
unique groups.

makechrootpkg.in

@@ -245,7 +245,7 @@ download_sources() {
 
 	local builddir
 	builddir="$(mktemp -d)"
-	chown "$makepkg_user:$makepkg_user" "$builddir"
+	chown "$makepkg_user:" "$builddir"
 
 	# Ensure sources are downloaded
 	sudo -u "$makepkg_user" --preserve-env=GNUPGHOME \