Example earthfile for testing weather script

[alexcb]

This draft PR shows how earthly can be used to test the script across different linux distros and http clients.

I picked the weather script to start with for this example, but should be extendable to all scripts if you think this is useful.

I renamed the shellcheck github action to ci.yml; and added the schellcheck logic under the "+lint" target. Tests can now be run via github actions, or locally using github.com/earthly/earthly

To run them locally, run:

earthly +all

or if you want to target a specific OS and http client combination, with:

earthly --build-arg OS=ubuntu --build-arg HTTP_CLIENT=httpie +test-weather

or if you just want to run linting:

earthly +lint