ci: compile the binaries only once

[darosior]

Currently, the functional tests on the CI are separated into 4 jobs (the 4 categories). Each job is going to compile the same revaultd binary as well as all the same test servers binaries (see #399). Doing the same computation 4 times is an obvious waste.
We should look into a way of having a first job compiling the servers, and then starting the 4 jobs running only the tests using those binaries.

[darosior]

Since #404 at least we don't recompile entirely all binaries.