Set VIRTUAL_ENV in pdm run

[pfmoore]

Pull Request Checklist

• 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.

If the environment is a virtualenv, pdm run will set the VIRTUAL_ENV environment variable, which lets tools know that a virtualenv is in use. This means that (for example) the py launcher on Windows correctly locates the environment's Python rather than using the system Python.

Fixes #1650

[pfmoore]

I'll look into adding some tests, but I need to work out how to get a test to choose "virtualenv" mode, global mode or pypackages mode when running pdm run first!

[codecov-commenter]

Codecov Report

Base: 85.20% // Head: 85.21% // Increases project coverage by +0.01% 🎉

Coverage data is based on head (62c6028) compared to base (d4eb11f).
Patch coverage: 42.85% of modified lines in pull request are covered.

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Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1652      +/-   ##
==========================================
+ Coverage   85.20%   85.21%   +0.01%     
==========================================
  Files          92       92              
  Lines        8555     8562       +7     
  Branches     1848     1851       +3     
==========================================
+ Hits         7289     7296       +7     
+ Misses        852      850       -2     
- Partials      414      416       +2     

Flag	Coverage Δ
unittests	85.03% <42.85%> (+0.01%)	⬆️

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

Impacted Files	Coverage Δ
src/pdm/cli/commands/run.py	90.98% <0.00%> (-0.76%)	⬇️
src/pdm/models/environment.py	66.66% <60.00%> (-0.19%)	⬇️
src/pdm/project/core.py	88.60% <0.00%> (+1.03%)	⬆️

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[frostming]

global mode or pypackages mode when running pdm run first!

nvm, i will take care of it