support setting env. vars in environments.yaml

[bcumming]

Add support for setting custom environment variables in a view definition in environments.yaml
https://spack.readthedocs.io/en/latest/env_vars_yaml.html
The syntax is based loosely on the env_vars.yaml format used by Spack

With some modifications to allow for effective ordering of environment variable updates.

example:

  views:
    nosu:
      link: roots
      exclude: [osu-micro-benchmarks]
      uenv:
        add_compilers: true
        prefix_paths:
          LD_LIBRARY_PATH: [lib, lib64]
        env_vars:
          set:
          - WOMBAT: "soup"
          - NOCOLOR: null # unset variable
          prepend_path:
          - PATH: "${@HOME@}/.local/x86_4/bin"
          append_path:
          - PATH: "${@HOME@}/.local/bin"

TODO

• update documentation