setup-timezone: force install virtual package

This is to avoid the message:
ERROR: You tried to add a non-repository package to system, but it would
 be lost on next reboot. Enable package caching (apk cache --help) or use
 --force if you know what you are doing.

setup-timezone.in

@@ -72,7 +72,7 @@ if $INSTALL_TZDATA; then
 	pkg=tzdata
 	apkdel=
 else
-	pkg="--virtual .setup-timezone tzdata"
+	pkg="--force --virtual .setup-timezone tzdata"
 	apkdel=".setup-timezone"
 fi