libtmux: test env fix

[tony]

uv add 'git+https://github.com/ppentchev/libtmux' --branch=pp-command-env

See also:

• uv add with --branch and a subdiretory doesn't check out the branch before checking the subdirectory astral-sh/uv#9604
• Fix passing both window command and environment libtmux#553

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 69.56%. Comparing base (fb16b3b) to head (8b87dd4).

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #955   +/-   ##
=======================================
  Coverage   69.56%   69.56%           
=======================================
  Files          26       26           
  Lines        1935     1935           
  Branches      365      365           
=======================================
  Hits         1346     1346           
  Misses        468      468           
  Partials      121      121           

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[tony]

#956