Speed up builds for COOP users with a CI builder and nix cache provider

[ross-spencer]

I'm working through the Tutorial for Orcfax, running nix develop #dev-tutorial has taken a while, do we have an anticipated build time for this section? It may be worth adding an approximation to the README as there's little feedback, though if it's not expected to be too high maybe not, it could be something my end.

It has been running here, 45 mins and counting.

System:

Intel® Core™ i7-8550U CPU @ 1.80GHz × 8 32GIB Memory.

NB. Providing they're configured correctly, this is with the IOHK pre-built binary config.