ipn/ipnlocal: add debug envknob for ACME directory URL (#14771)

Adds an envknob setting for changing the client's ACME directory URL.
This allows testing cert issuing against LE's staging environment, as
well as enabling local-only test environments, which is useful for
avoiding the production rate limits in test and development scenarios.

Fixes #14761

Change-Id: I191c840c0ca143a20e4fa54ea3b2f9b7cbfc889f
Signed-off-by: Tom Proctor <tomhjp@users.noreply.github.com>

ipn/ipnlocal/cert.go

@@ -659,8 +659,9 @@ func acmeClient(cs certStore) (*acme.Client, error) {
 	// LetsEncrypt), we should make sure that they support ARI extension (see
 	// shouldStartDomainRenewalARI).
 	return &acme.Client{
-		Key:       key,
-		UserAgent: "tailscaled/" + version.Long(),
+		Key:          key,
+		UserAgent:    "tailscaled/" + version.Long(),
+		DirectoryURL: envknob.String("TS_DEBUG_ACME_DIRECTORY_URL"),
 	}, nil
 }