ink_e2e: Compile times too long when using `additional_contracts`

[athei]

The compile times for incremental changes explode when using ink_e2e with additional_contracts. This makes writing tests really annoying. Reason for that is that every time the test is modified it triggers an additional run of cargo contract on the additional contracts. Together with https://github.com/paritytech/cargo-contract/issues/926 this compounds to a multi second wait time.

The macro needs to skip the build step if the additional contract wasn't modified.