Reduce the number of Spack environments

[bcumming]

Remove a long-standing work around to an old Spack bug, that required us to generate a separate Spack environment for every view.
This meant that if an environments.yaml recipe contained more than one view in an environment (e.g. default and develop), then the environment would be concretised twice.

Spack 1.0 fixes this issue, so we can .

This has no impact on users or recipes, it is a back end optimization.

Fixes #209