Add ${build-number}

[ghost]

Add a ${build-number} variable which expands to the number of builds started. This provides a way to run a command that is run every time a build is started (#255).

[rgrinberg]

Can we have something like a @phony alias for user to depend on? I think that having to include ${build-number} in some action is a bit unintuitive. Also, I'm not sure it's such a good idea to expose this number to users either, as in your suggested workaround in #255 (comment)

[ghost]

Actually, maybe the best is to be explicit and allow to say that a rule has unspecified dependencies. In the end that's what it is. Then in polling mode Dune could decide to re-run it periodically for instance. We can add a field (has_unspecified_deps) to rule and alias stanzas.