Attempt at resolving errors in building test dependencies

[matt-graham]

May resolve #292

An initial attempt at fixing the issues in building fftw as part of building wheels for pyssht and so3 by defining an environment variable CMAKE_POLICY_VERSION_MINIMUM=3.5 as recommended in associated CMake error.

Also applies similar change to #284 in actions/checkout step in publish workflow to tests workflow to ensure tags are checked out and so setuptools_scm correctly infers package version when building / installing s2fft.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 96.29%. Comparing base (6d55715) to head (33bcbd8).
Report is 3 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #293   +/-   ##
=======================================
  Coverage   96.29%   96.29%           
=======================================
  Files          31       31           
  Lines        3586     3586           
=======================================
  Hits         3453     3453           
  Misses        133      133           

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