`spack install` previously concretized, writes the entire environment
out, regenerated views, then wrote and regenerated views
again. Regenerating views is slow, so ensure that we only do that once.
- [x] add an option to env.write() to skip view regeneration
- [x] add a note on whether regenerate_views() shouldn't just be a
separate operation -- not clear if we want to keep it as part of write
to ensure consistency, or take it out to avoid performance issues.
c83e365c59