Speed up CI by re-using relayer binary for E2E tests

[romac]

We could likely significantly speed up the end-to-end test job by copying the relayer binary built as part of the Rust job into the image instead of rebuilding it.

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[adizere]

The original CI infrastructure was done as part of informalsystems/ibc-rs#544.
This was later extended with comprehensive tests in informalsystems/ibc-rs#571.

The work in the original CI consists of three docker images:

• two of them were gaia chain images called "informaldev/ibc-0" and "informaldev/ibc-1"
• the third image is for the relayer, and is dependent on the gaia images; this is built on-the-fly upon every commit to a pull-request

There are two basic differences between the original work and the new PR:

1. the relayer binary is now compiled as part of the Github action, and then this binary is made available (cp) to the relayer docker image:

https://github.com/informalsystems/ibc-rs/blob/81af0b0b0418c4a365617e3cbf6006db80a3c831/.github/workflows/e2e.yaml#L13-L17

in the original work, the binary was compiled directly inside the docker-compose task:

https://github.com/informalsystems/ibc-rs/blob/148e196308a217542f2dc49bfe5881362bf1013d/ci/relayer.Dockerfile#L28

2. the cargo build command for obtaining the relayer binary no longer relies on --all --release, shortening the build cycle by ~5 minutes (almost halved)