Figure out a nice way to integrate with CI

[szg251]

We've been using process-compose in Hercules CI for tx-village ,but it has a few quirks:

• we need a new derivation for the tests https://github.com/mlabs-haskell/tx-village/blob/main/extras/tx-indexer-testsuite/build.nix#L45-L67 , the current repo should provide this
• executables must be built by nix (no cargo or cabal works). This is a bit cumbersome if we want to use the environment for local testing: we need to direnv reload each time before rerunning the tests

I'm still not convinced if this should be solved by this repo, or perhaps flake-lang.nix.