Trial GitHub Actions vs Cirrus CI

[trexfeathers]

📰 Custom Issue

@ocefpaf has correctly pointed out (SciTools/cf-units#175, SciTools/nc-time-axis#66) that GHA is the most popular CI implementation for scientific Python projects. Given that barriers to involvement are a big factor in the health of a project: we need a really good reason to go against the crowd as we do now using Cirrus CI.

I believe our reasons for using Cirrus are to minimise the runtime for a full CI run. We should therefore do a trial to see if it possible to get similar/faster runtime using GHA instead. I'm told that GHA also has caching solutions like Cirrus does, and the minutes are free for public repositories so there should be no trouble with parallelism.

[trexfeathers]

I probably shouldn't have been so quick to shoot down @jamesp's idea on SciTools/iris-grib#242!

[rcomer]

Cartopy went to GHA in December SciTools/cartopy#1691