buildrepo: fix purging of old packages

buildrepo.in

@@ -81,7 +81,7 @@ build() {
 	if [ -n "$dopurge" ]; then
 		local tmp=$(mktemp /tmp/$program-XXXXXX)
 		local purgefiles
-		cd "$repodir/$1" || return 1
+		cd "$repodir/$repo/$CARCH" || return 1
 		trap 'rm -f "$tmp"; exit 1' INT
 		( listpackages "$1") >$tmp
 		purge=$(ls *.apk 2>/dev/null | grep -v -w -f $tmp)