Allow disabling automatic fallback to 8.8.8.8 and 1.1.1.1

[AkihiroSuda]

lima/pkg/hostagent/dns/dns.go

Line 24 in b121229

var defaultFallbackIPs = []string{"8.8.8.8", "1.1.1.1"}

We should probably have an env var like LIMA_DISABLE_FALLBACK_DNS=1.
I also wonder if there is some "standard" env var to disable these well-known fallback DNS.

Similar to:

• [EU GDPR] [Feature Request] Fallback-DNS (Google) should be removed as it is incompatible with GDPR, PCI-DSS, most company policies, doesn't follow secure coding practices etc. moby/moby#47541

[smcavoy]

would moving to Quad9 be sufficient?
Also, is Clouldflare's 1.1.1.1 compliant?