[e2e] slow CI runs

[rnbguy]

I noticed, the CI runs are rerunning some build steps at every job -- which makes the runs slow.

In #120 I cached the wasm light client build step and it works fine. We still need to figure out why

• cairo contract build step rebuilds the cairo binaries and recompiles the cairo contracts
• integration tests step rebuilds the relayer crates from scratch

The integration test runs for ~2 mins, but it ends up being ~9 min for rerunning build steps. nix with cachix should cache all the build steps. We should look into this after 2nd demo.

[rnbguy]

I found a cairo nix overlay here: https://docs.swmansion.com/scarb/download#nixos

[rnbguy]

Probably, we can use crate2nix or naersk

Ref: https://discourse.nixos.org/t/rustplatform-buildrustpackage-and-caching/11138

Also, I think rust-cache doesn't really work with Nix.