Add flag to disable bootstrap dns

Adds an optional (default off) boolean environment flag
`TS_NO_DNS_FALLBACK` which will simply disable functionality to fallback
to using Tailscale DNS servers as indicated in text logs like:

```
trying bootstrapDNS("derp8c.tailscale.com", "2a03:b0c0:1:d0::e1f:4001")
for "log.tailscale.io" ...
```

For some user environments, it is desirable to strictly control outbound
access to external servers and and leaked data or metadata - and losing
this functionality is a desired trade off.

Closes #7981

Signed-off-by: Kevin Allen <kallen@bostondynamics.com>

net/dnscache/dnscache.go

@@ -169,6 +169,7 @@ func (r *Resolver) ttl() time.Duration {
 }
 
 var debug = envknob.RegisterBool("TS_DEBUG_DNS_CACHE")
+var noDNSFallback = envknob.RegisterBool("TS_NO_DNS_FALLBACK")
 
 // debugLogging allows enabling debug logging at runtime, via
 // SetDebugLoggingEnabled.
@@ -302,7 +303,7 @@ func (r *Resolver) lookupIP(host string) (ip, ip6 netip.Addr, allIPs []netip.Add
 			ips, err = resolver.LookupNetIP(ctx, "ip", host)
 		}
 	}
-	if (err != nil || len(ips) == 0) && r.LookupIPFallback != nil {
+	if (err != nil || len(ips) == 0) && r.LookupIPFallback != nil && !noDNSFallback() {
 		ctx, cancel := context.WithTimeout(context.Background(), 30*time.Second)
 		defer cancel()
 		if err != nil {
@@ -451,6 +452,12 @@ func (d *dialer) shouldTryBootstrap(ctx context.Context, err error, dc *dialCall
 		d.dnsCache.dlogf("not using bootstrap DNS: no fallback")
 		return false
 	}
+	if noDNSFallback() {
+		if debug() {
+			log.Printf("dnscache: not using bootstrap DNS: disabled via TS_NO_DNS_FALLBACK")
+		}
+		return false
+	}
 
 	// We can't retry if the context is canceled, since any further
 	// operations with this context will fail.