feature/featuretags, all: add build features, use existing ones in more places

Saves 270 KB.

Updates #12614

Change-Id: I4c3fe06d32c49edb3a4bb0758a8617d83f291cf5
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

util/cloudenv/cloudenv.go

@@ -16,6 +16,7 @@ import (
 	"strings"
 	"time"
 
+	"tailscale.com/feature/buildfeatures"
 	"tailscale.com/syncs"
 	"tailscale.com/types/lazy"
 )
@@ -51,6 +52,9 @@ const (
 // ResolverIP returns the cloud host's recursive DNS server or the
 // empty string if not available.
 func (c Cloud) ResolverIP() string {
+	if !buildfeatures.HasCloud {
+		return ""
+	}
 	switch c {
 	case GCP:
 		return GoogleMetadataAndDNSIP
@@ -92,6 +96,9 @@ var cloudAtomic syncs.AtomicValue[Cloud]
 
 // Get returns the current cloud, or the empty string if unknown.
 func Get() Cloud {
+	if !buildfeatures.HasCloud {
+		return ""
+	}
 	if c, ok := cloudAtomic.LoadOk(); ok {
 		return c
 	}