Only run CI on master and develop

The current build.yml runs on pushes to all branches, including the gh-pages branch which only contains the docs and no actual code, and will therefore always fail CI. It makes more sense to run it only on the "main" branches (master and develop), especially given the resources consumed by the CI runs.

I also updated the CI workflow names so they're not just all called "Python" - this will make debugging easier if things break on specific python/gdal versions.