Figure out a proper way to depend on a EPNix support top

[minijackson]

Currently, you can't just use the flake build as a support top dependency, because the support top might be compiled with a different version of epics-base, which gets refused at compilation time by the "checkRelease" step.

What we should do, instead of using the exposed build, is to use the exposed derivation function, which takes epics-base and other dependencies as argument.

cc @stephane-cea

[minijackson]

Fixed by #55