Improve matrixing in GHA to work with SciTools/cartopy#2303

[trexfeathers]

Rationale

When a test fails, image results are uploaded. Since actions/upload-artifact@v4 (#2303), it is the responsibility of the workflow YAML file to avoid name clashes in the artifacts. This PR makes sure that the whole matrix is included in the artifact name and path, avoiding clashes between matrix.shapely-dev true vs false.

Implications

Failures for ubuntu-latest, latest python will not result in one of the image uploads failing due to a name clash with the other.

[trexfeathers]

Proof of concept - successful image uploads both here and here.

[trexfeathers]

Thanks, people!