Run CI on both 1.6 and the latest stable Julia; only run CI on Linux; various other tweaks to the CI workflow file

[DilumAluthge]

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[codecov]

Codecov Report

Merging #12 (5816a9e) into main (326f605) will not change coverage.
The diff coverage is n/a.

❗ Current head 5816a9e differs from pull request most recent head c462aa0. Consider uploading reports for the commit c462aa0 to get more accurate results

@@            Coverage Diff            @@
##              main       #12   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            1         1           
  Lines           26        26           
=========================================
  Hits            26        26           

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