CI: Don't run the test suite on tags (but do build the docs on tags)

[DilumAluthge]

There's no need to run the test suite on tags, because before we make a release, we run the test suite on master.

We do need to continue building the docs on tags.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 87.73%. Comparing base (9bef815) to head (143853e).
Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1009   +/-   ##
=======================================
  Coverage   87.73%   87.73%           
=======================================
  Files           3        3           
  Lines         799      799           
=======================================
  Hits          701      701           
  Misses         98       98           

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