Allow bridge relayers to be built from repo (#3069)

* Bump Rococo/Wococo `spec_version` used by the bridge relayer

* Prevent bridge relayer from being part of workspace

This allows us to build the relayer without pulling in all its
dependencies into the Polkadot repo by indicating its manifest path when
building.

* Revert "Bump Rococo/Wococo `spec_version` used by the bridge relayer"

This reverts commit 90fdeecbba2ed2a75fc220dbc14fb4fa792dfc6e.