CI: Simplify actions to build once, test many

[effigies]

This PR reduces a lot of duplicated testing in GHA.

1. Build the packages once and only once.
2. Test version strings and pytest across various installation methods.

[codecov-commenter]

Codecov Report

Base: 70.37% // Head: 70.37% // No change to project coverage 👍

Coverage data is based on head (4f709f2) compared to base (ad4401c).
Patch has no changes to coverable lines.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #304   +/-   ##
=======================================
  Coverage   70.37%   70.37%           
=======================================
  Files          17       17           
  Lines        1134     1134           
  Branches      189      192    +3     
=======================================
  Hits          798      798           
  Misses        274      274           
  Partials       62       62           

Flag	Coverage Δ
ds005	60.58% <ø> (ø)
ds054	51.23% <ø> (ø)

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[effigies]

@mgxd Any opposition to this?