lima: avoid mutating user config when injecting default DNS host

[ousamabenyounes]

Problem

newConf in environment/vm/lima/yaml.go builds the lima config from the colima
config. It assigned the user's DNS-hosts map by reference and then injected
colima's internal host.docker.internal default into it:

l.HostResolver.Hosts = conf.Network.DNSHosts // aliases the caller's map
...
if _, ok := l.HostResolver.Hosts["host.docker.internal"]; !ok {
    l.HostResolver.Hosts["host.docker.internal"] = "host.lima.internal"
}

Because Go maps are reference types, mutating l.HostResolver.Hosts also mutates
the caller's conf.Network.DNSHosts. That same config object is later saved
(l.conf = conf → SaveToFile, and the CLI's configmanager.Save shares the
same underlying map), so any user who has at least one dnsHosts entry ends up
with an unwanted host.docker.internal: host.lima.internal line silently written
into their colima.yaml.

When conf.Network.DNSHosts was nil the code happened to allocate a fresh map,
so the leak only affected users who set their own DNS hosts.

Fix

Copy the user-provided hosts into a new map before injecting the internal default,
so the caller's config is never mutated. Behavior is otherwise unchanged: the
host.docker.internal default is still added when absent, and a user override is
still respected.

Test verification (RED → GREEN)

Added Test_newConf_doesNotMutateDNSHosts, which calls newConf with a
user-supplied dnsHosts map and asserts the original map is left untouched.

RED — on the unmodified base (bug present):

=== RUN   Test_newConf_doesNotMutateDNSHosts
    yaml_test.go:121: newConf mutated caller DNSHosts, injected host.docker.internal: map[host.docker.internal:host.lima.internal myhost.internal:1.2.3.4]
    yaml_test.go:124: newConf mutated caller DNSHosts size, got 2 entries: map[host.docker.internal:host.lima.internal myhost.internal:1.2.3.4]
--- FAIL: Test_newConf_doesNotMutateDNSHosts (0.00s)
FAIL	github.com/abiosoft/colima/environment/vm/lima

GREEN — with the fix:

=== RUN   Test_newConf_doesNotMutateDNSHosts
--- PASS: Test_newConf_doesNotMutateDNSHosts (0.00s)
ok  	github.com/abiosoft/colima/environment/vm/lima

go build ./..., go test ./..., and golangci-lint run (v2.11.3) are all green.