fix: in project conda venv activate without argument

[ProgramRipper]

Pull Request Checklist

• A news fragment is added in news/ describing what is new.
  - [ ] Test cases added for changed code. (current tests do not cover venv activate without env)

Describe what you have changed in this PR.

For in project venv created by conda, using pdm venv activate without specifying env_name would be a failure:

(base) programripper:~/conda-test$ echo $CONDA_PREFIX  # Actually it is quite common for conda that isn't installed normally (e.g. via pyenv) that not to have CONDA_PREFIX properly set, which will make pdm fail faster.
/home/programripper/.pyenv/shims/conda
(base) programripper:~/conda-test$ pdm venv list
Virtualenvs created with this project:

*  in-project: /home/programripper/conda-test/.venv
(base) programripper:~/conda-test$ cat .pdm.toml
[venv]
backend = "conda"

[python]
path = "/home/programripper/conda-test/.venv/bin/python"
(base) programripper:~/conda-test$ pdm venv activate
Can't activate a non-venv Python /home/programripper/conda-test/.venv/bin/python, you can specify one with pdm venv activate <env_name>

By this PR, user can activate in project venv created by conda using pdm venv activate as venv created by venv or virtualvenv.

[codecov-commenter]

Codecov Report

Base: 85.20% // Head: 85.11% // Decreases project coverage by -0.09% ⚠️

Coverage data is based on head (ce3a74b) compared to base (4751b40).
Patch coverage: 60.00% of modified lines in pull request are covered.

📣 This organization is not using Codecov’s GitHub App Integration. We recommend you install it so Codecov can continue to function properly for your repositories. Learn more

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1735      +/-   ##
==========================================
- Coverage   85.20%   85.11%   -0.09%     
==========================================
  Files          92       92              
  Lines        8638     8661      +23     
  Branches     1865     1871       +6     
==========================================
+ Hits         7360     7372      +12     
- Misses        858      867       +9     
- Partials      420      422       +2     

Flag	Coverage Δ
unittests	84.93% <60.00%> (-0.09%)	⬇️

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

Impacted Files	Coverage Δ
src/pdm/utils.py	85.82% <60.00%> (-0.65%)	⬇️
src/pdm/installers/manager.py	86.66% <0.00%> (-13.34%)	⬇️
src/pdm/models/project_info.py	92.85% <0.00%> (-1.43%)	⬇️
src/pdm/installers/synchronizers.py	83.33% <0.00%> (-1.20%)	⬇️
src/pdm/cli/commands/show.py	85.10% <0.00%> (-0.32%)	⬇️

Help us with your feedback. Take ten seconds to tell us how you rate us. Have a feature suggestion? Share it here.

☔ View full report at Codecov.
📢 Do you have feedback about the report comment? Let us know in this issue.