When a controlclient receives non-keepalive netmap, it updates the controlknobs
based on the capability map of the self node. We added a new knob based on
tailcfg.NodeAttrCacheNetworkMaps in be2f554dd3, and this ensures we correctly
propagate the attribute to the knob when coming up from a cache as well.
Updates #12639
Updates tailscale/projects#27
Change-Id: I40d34053c9743757f9fddca715379fc63a9ae6a0
Signed-off-by: M. J. Fromberger <fromberger@tailscale.com>