Bootstrap logic in proxy-dns mode

There is a flag `bootstrap` and its corresponding env var `TUNNEL_DNS_BOOTSTRAP` with a sane default value:

https://github.com/cloudflare/cloudflared/blob/00cd7c333ce7e7493a3dff245aa79b0cbbd77090/cmd/cloudflared/proxydns/cmd.go#L50-L55

As I understand it, bootstraps are used to resolve domains in upstream urls.

So I set `TUNNEL_DNS_UPSTREAM=https://dns.digitale-gesellschaft.ch/dns-query` and start `cloudflared-linux-arm64 proxy-dns`. On my test system outgoing traffic is only allowed to tcp port 443. When I try to resolve a name there is such message in log:

```
ERR failed to connect to an HTTPS backend "https://dns.digitale-gesellschaft.ch/dns-query"
error="failed to perform an HTTPS request: Post \"https://dns.digitale-gesellschaft.ch/dns-query\":
dial tcp: lookup dns.digitale-gesellschaft.ch on 192.168.2.1:53:
write udp 192.168.2.3:44618->192.168.2.1:53: write: operation not permitted"
```

`192.168.2.1:53` is the system resolver. `192.168.2.3` is the test system.

So instead of falling back to the secure bootstrap, it tried to use the cleartext system resolver. Should it be like that?