Building the Collator builds all the Polkadot runtimes.

[JoshOrndorff]

When building the test collator with cargo build --release -p rococo-validator, all polkadot runtimes are built. For example it builds kusama-runtime, westend-runtime, polkadot-test-runtime, etc.

This is harmless in terms of correctness, but it adds a lot of build time.

[xlc]

Currently the upward-message crate depends on all four runtimes. The relay chain runtime should be feature gated so we don't need to pull & build unrelated code.