Integrate runIf functionality into mkEffect

[roberth]

The dependencies attribute usually covers the prebuilt one anyway, so we don't gain much from the extra complexity of multiple attrs.

Plan: two parameters, a low-level one and a high-level one. The low level one controls whether mkEffect returns an effect or a dependency-only "build". The high level one has a default implementation in terms of the low level one, but is intended to be altered by effect functions like nixopsDeploy to perform a networked "read-only" check instead of a deployment.

Goal: the way of making things conditional carries over between all effects.