ipn/ipnlocal: skip writing netmaps to disk when disabled (#18883)

We use the TS_USE_CACHED_NETMAP knob to condition loading a cached netmap, but
were hitherto writing the map out to disk even when it was disabled. Let's not
do that; the two should travel together.

Updates #12639

Change-Id: Iee5aa828e2c59937d5b95093ea1ac26c9536721e
Signed-off-by: M. J. Fromberger <fromberger@tailscale.com>

ipn/ipnlocal/local.go

@@ -6241,8 +6241,10 @@ func (b *LocalBackend) setNetMapLocked(nm *netmap.NetworkMap) {
 	var login string
 	if nm != nil {
 		login = cmp.Or(profileFromView(nm.UserProfiles[nm.User()]).LoginName, "<missing-profile>")
-		if err := b.writeNetmapToDiskLocked(nm); err != nil {
-			b.logf("write netmap to cache: %v", err)
+		if envknob.Bool("TS_USE_CACHED_NETMAP") {
+			if err := b.writeNetmapToDiskLocked(nm); err != nil {
+				b.logf("write netmap to cache: %v", err)
+			}
 		}
 	}
 	b.currentNode().SetNetMap(nm)