ipn/store, feature/condregister: permit callers to empty import optonal ipn stores

This permits other programs (in other repos) to conditionally
import ipn/store/awsstore and/or ipn/store/kubestore and have them
register themselves, rather than feature/condregister doing it.

Updates tailscale/corp#32922

Change-Id: I2936229ce37fd2acf9be5bf5254d4a262d090ec1
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

ipn/store/awsstore/store_aws_test.go

@@ -1,7 +1,7 @@
 // Copyright (c) Tailscale Inc & AUTHORS
 // SPDX-License-Identifier: BSD-3-Clause
 
-//go:build linux && !ts_omit_aws
+//go:build !ts_omit_aws
 
 package awsstore