Validate environment paths

[sersorrel]

to avoid breaking the world if someone creates an environment called users/not-a-username

[codecov]

Codecov Report

Attention: 34 lines in your changes are missing coverage. Please review.

Comparison is base (16e49df) 65.29% compared to head (d934932) 64.96%.
Report is 7 commits behind head on develop.

Files	Patch %	Lines
softpack_core/schemas/environment.py	23.68%	29 Missing ⚠️
softpack_core/artifacts.py	76.92%	3 Missing ⚠️
softpack_core/schemas/groups.py	86.66%	2 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop      #44      +/-   ##
===========================================
- Coverage    65.29%   64.96%   -0.33%     
===========================================
  Files           16       17       +1     
  Lines          775      822      +47     
  Branches       131      143      +12     
===========================================
+ Hits           506      534      +28     
- Misses         262      281      +19     
  Partials         7        7              

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[mjkw31]

LGTM.