Fix offline builds

[ascjones]

cargo contract build should work when not connected to the network.

[cmichi]

Whoever fixes this should also add a CI stage that tests cargo contract build --offline without networking.

[ltfschoen]

@ascjones is this what you were after? #1135

[ascjones]

@ascjones is this what you were after? #1135

No, I don't think it is necessary to detect network status.

I believe this issue was resolved by adding the --offline flag: #356, which passes the flag through to cargo.