CN server: use local switches for installation instructions and in CI

[samcowger]

Using switches makes the process a bit less fiddly and error-prone, and should obviate some of the stale dependency issues the previous process entailed.

[samcowger]

Aha - local switches also fall prey to the problem of Cerberus's runtime file dependency that I mentioned in #130 (comment). Although these instructions ought to work well for building a server in isolation, they'd also need to tell users how/where to install those runtime files and how to point the server at them - unless we end up taking one of the other paths I mentioned in that comment.

[samcowger]

Aha - local switches also fall prey to the problem of Cerberus's runtime file dependency that I mentioned in #130 (comment). Although these instructions ought to work well for building a server in isolation, they'd also need to tell users how/where to install those runtime files and how to point the server at them - unless we end up taking one of the other paths I mentioned in that comment.

I've gotten around this for now by having a user point the client at the specific opam switch into which they've installed the server, since that switch will also contain the Cerberus runtime. Since this amounts to pointing the client at the right binary and runtime files, it should be possible to use a similar approach for binary distributions in the future.