Add pixi to excluded directories

[maresb]

Pixi is a recent tool that manages project-specific Conda environments. The environments are created in a cache directory named .pixi/ under the project root. They should never be included in a Python distribution. For additional discussion, see https://github.com/prefix-dev/pixi/issues/2115#issuecomment-2379136870

[maresb]

Rebasing on master without changes for a linear history

[maresb]

Looks like upload-artifact failed, probably just a random glitch. I don't seem to have permission to rerun the task, so I will reopen.

 Upload coverage data
Failed to FinalizeArtifact: Failed to make request after 5 attempts: Request timeout: /twirp/github.actions.results.api.v1.ArtifactService/FinalizeArtifact