Be able to `(install` something from another context

[dinosaure]

Probably related to #5592 (and the ability to use artifacts across contexts) but it will be interesting to tell to dune to install an artifact which is only available from a specific context such as:

(rule
 (target unikernel.hvt)
 (enabled_if (= %{context_name} "solo5"))
 (deps unikernel/main.exe)
 (action (bash "strip %{deps} -o %{target}")))

(install
 (files unikernel.hvt)
 (enabled_if (= %{context_name} "solo5"))
 (section bin)
 (package my-unikernel))

As far as I can tell, the produced my-unikernel.install mentions only artifacts from the default context. From the example above, dune also complains that unikernel.hvt does not exist for @default and @all (which is... expected?).

[dinosaure]

It seems that I missed the my-unikernel-solo5.install file, but I am not sure that opam (in my case) reacts correctly with such a file if it is to install my-unikernel: a test confirms that opam only takes my-unikernel.install into account (so unikernel.hvt is not installed).