archbuild: Store chroots in /var/lib instead of /var/tmp

/var/tmp is cleaned up by tmpfiels by default which we cannot handle gracefully.

archbuild.in

@@ -14,7 +14,7 @@ else
 	repo=${tag%-*}
 	arch=${tag##*-}
 fi
-chroots='/var/tmp/archbuild'
+chroots='/var/lib/archbuild'
 clean_first=false
 
 usage() {