Updated substrate-relay version for tests (#5017)

## Testing

Both Bridges zombienet tests passed, e.g.:
https://gitlab.parity.io/parity/mirrors/polkadot-sdk/-/jobs/6698640
https://gitlab.parity.io/parity/mirrors/polkadot-sdk/-/jobs/6698641
https://gitlab.parity.io/parity/mirrors/polkadot-sdk/-/jobs/6700072
https://gitlab.parity.io/parity/mirrors/polkadot-sdk/-/jobs/6700073

docker/dockerfiles/bridges_zombienet_tests_injected.Dockerfile

 # this image is built on top of existing Zombienet image
 ARG ZOMBIENET_IMAGE
 # this image uses substrate-relay image built elsewhere
-ARG SUBSTRATE_RELAY_IMAGE=docker.io/paritytech/substrate-relay:v1.6.5
+ARG SUBSTRATE_RELAY_IMAGE=docker.io/paritytech/substrate-relay:v1.6.6
 
 # metadata
 ARG VCS_REF