fix build steps

[quinlanj]

The most recent distribution of core-crypto assumes the presence of a code coverage script scripts/code-coverage.cmake. This causes cmake to fail and isn't really relevant for our use.

I've changed the build steps to include removing references to code-coverage. Everything builds successfully after that.

[Curve]

Small note for arch linux users: You can also install libcorecrypto-git from the aur - which automatically applies the required fixes mentioned here