Fix `lorri direnv` triggering an unconditional rebuild every time it is run. After fixing up the build loop people suddenly started noticing that lorri was evaluating every time something ran `lorri direnv`, which could potentially be every time the user switched between buffers in the editor. This is not the intended behaviour, since we should run an unconditional build only the first time the project is added to the watcher, and after rely on the watcher to notify us of any file changes (or the user running `lorri internal ping` to force a rebuild).