CI runtime improvements

[steveej]

• #93 proposes to use the nix built hc-scaffold for the integration tests.

• #95 proposes running cargo test. instead of cargo test we could add a crane-based test derivation so the dependencies could be shared between the tests and the CLI. this assume we can agree on using a common cargo profile. any objections to using "release" for the test derivation?

• the script run_tests.sh could be parallelized using a matrix on github actions.