CI: Test JupyterLite build even if not deploying.

[PapyChacal]

Following #328. This makes the CI build the JupyterLite distribution with every other test and still deploy it only on pushes to the main branch.
Otherwise, issues affecting the JupyterLite can only be found after a merge..

[codecov]

Codecov Report

Base: 88.87% // Head: 88.87% // No change to project coverage 👍

Coverage data is based on head (259f44f) compared to base (fcc7ac4).
Patch has no changes to coverable lines.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #335   +/-   ##
=======================================
  Coverage   88.87%   88.87%           
=======================================
  Files          60       60           
  Lines        7595     7595           
  Branches     1238     1238           
=======================================
  Hits         6750     6750           
  Misses        591      591           
  Partials      254      254           

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[math-fehr]

Nice!