Allow clients to force netmap caching via the TS_FORCE_CACHE_NETMAP
environment variable, bypassing the requirement for the control server
to grant the cache-network-maps node capability.
This is useful for tsnet users who want faster startup times via
cached netmaps but whose control plane doesn't grant the capability.
Change-Id: I447b57311940d51a5ce9021236add48e0333d0b5
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>