Update CI settings to cancel jobs when PR is closed

[bsipocz]

Solution to issue cannot be found in the documentation.

• I checked the documentation.

Issue

The duplicate PR (conda-forge/gammapy-feedstock#30) has been closed very soon it was opened, yet none of the CI runs got canceled, and instead, they were let run and finish. Would be great to auto-cancel them if possible.

Installed packages

N/A

Environment info

N/A