Explicitly install pip in the unit test workflow

[KathleenLabrie]

Maybe pip used to be installed automatically by conda but it isn't anymore. Explicitly ensure that pip is installed before continuing.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 66.78%. Comparing base (c84374f) to head (68df07f).

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #521   +/-   ##
=======================================
  Coverage   66.78%   66.78%           
=======================================
  Files         318      318           
  Lines       40383    40383           
=======================================
  Hits        26970    26970           
  Misses      13413    13413           

Flag	Coverage Δ
f2	29.93% <ø> (ø)
ghost	30.47% <ø> (ø)
ghost_integ	31.46% <ø> (ø)
gmos	30.45% <ø> (ø)
gmosls	36.74% <ø> (ø)
gnirs	31.62% <ø> (ø)
gsaoi	24.38% <ø> (ø)
niri	28.22% <ø> (ø)
regression	33.73% <ø> (ø)
slow	45.13% <ø> (ø)
unit	49.14% <ø> (ø)
wavecal	25.51% <ø> (ø)

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

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.