Refactor: Extract environment related code from Candidate

[frostming]

Pull Request Check List

• A news fragment is added in news/ describing what is new.
• Test cases added for changed code.

Describe what you have changed in this PR.

[codecov-commenter]

Codecov Report

Merging #920 (0e96838) into main (ef0ddee) will decrease coverage by 0.69%.
The diff coverage is 89.87%.

@@            Coverage Diff             @@
##             main     #920      +/-   ##
==========================================
- Coverage   84.17%   83.48%   -0.70%     
==========================================
  Files          74       74              
  Lines        6447     6472      +25     
  Branches     1333     1340       +7     
==========================================
- Hits         5427     5403      -24     
- Misses        699      751      +52     
+ Partials      321      318       -3     

Flag	Coverage Δ
unittests	83.25% <89.87%> (-0.70%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
pdm/installers/synchronizers.py	84.54% <ø> (-2.28%)	⬇️
pdm/utils.py	75.71% <50.00%> (-2.63%)	⬇️
pdm/models/candidates.py	89.55% <90.62%> (-0.21%)	⬇️
pdm/cli/actions.py	79.94% <100.00%> (+0.11%)	⬆️
pdm/cli/commands/show.py	85.71% <100.00%> (ø)
pdm/cli/utils.py	80.06% <100.00%> (-1.25%)	⬇️
pdm/formats/requirements.py	91.39% <100.00%> (ø)
pdm/installers/manager.py	85.71% <100.00%> (+0.52%)	⬆️
pdm/models/repositories.py	77.63% <100.00%> (ø)
pdm/project/core.py	89.91% <100.00%> (ø)
... and 10 more

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