Simplify conda use in tests

[dstansby]

This:

• Sets the defuault shell to avoid specifying it at every step
• Doesn't create a new environment, but uses the default one provided by setup-miniconda
• Puts back the install for pcodec on all Python versions, since wheels are out now

This simplifies the setup a fair bit, and cuts the total testing time by ~50% (9mins total vs 18 mins total for another recent build).

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.92%. Comparing base (c5cf0c5) to head (44d6f57).
Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #674      +/-   ##
==========================================
- Coverage   99.92%   99.92%   -0.01%     
==========================================
  Files          62       62              
  Lines        2754     2752       -2     
==========================================
- Hits         2752     2750       -2     
  Misses          2        2              

Files with missing lines	Coverage Δ
numcodecs/tests/test_zarr3.py	100.00% <ø> (ø)