This adds a new configuration section called `env_vars:` that can be set in an environment.
It looks very similar to the existing `environment:` section that can be added to `modules.yaml`,
but it is global for an entire spack environment. It's called `env_vars:` to deconflate it with spack
environments (the term was too overloaded).
The syntax looks like this:
```yaml
spack:
specs:
- cmake%gcc
env_vars:
set:
ENVAR_SET_IN_ENV_LOAD: "True"
```
Any of our standard environment modifications can be added to the `env_vars` section, e.g.
`prepend_path:`, `unset:`, `append_path:`, etc. Operations in `env_vars:` are performed
on `spack env activate` and undone on `spack env deactivate`.
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