generate a single spack environment for each environment in `environments.yaml`

[bcumming]

Spack before v0.21 had a limitation in that it would only generate environment activation scripts for views with the name default.

Stackinator generates these activation scripts to scrape for environment variables to set when views are loaded.

Because an environment can have multiple views, we had to replicate an environment for each view:

https://github.com/eth-cscs/stackinator/blob/master/stackinator/recipe.py#L335-L400

This is no longer needed, and should be fixed.

• it complicates the implementation
• it creates a lot more output during concretization and building of the uenv
• it has been observed that sometimes the same environment with different views will concretize differently (why, we don't know)