Also, fix the case where we didn't use --network with EXPOSE_TCP,
as reported by @theborakompanioni:
```
I get Wrong network! Our Bitcoin backend is running on 'regtest', but we expect 'main'. with LIGHTNINGD_NETWORK := regtest when param --network is not provided.
```
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>