Is your feature request related to a problem? Please describe.

This is a comment from the operational folks at Fleet Numerical:

If one is only dealing with the development of one application then the .spack directory can be whatever it is. However, for us we have to coordinate many applications and in the future other modules that use spack-spack that may have conflicts for the library user [who may use spack without spack-stack, too]. It means that we will have to remove the .spack directory with every build of an application.

Describe the solution you'd like

Isolate spack-stack environments from any existing user configuration of spack, typically in ~/.spack/

Additional context

We discussed this a while ago but decided against it, because we didn't have a good way and good documentation to create bootstrap mirrors for air-gapped systems. This was fixed in #1468 a while ago.