feat(publish): skip build step with `--skip-existing`

[lucascolley]

https://pixi.prefix.dev/latest/reference/cli/pixi/publish/#arg---skip-existing is documented to

Skip uploading packages that already exist on the target channel.

My understanding of https://rattler-build.prefix.dev/latest/reference/cli/rattler-build/publish/#arg---skip-existing, however, is that it will also skip building packages, if they would be skipped at the upload stage. It would be good if pixi publish had the same behaviour, to avoid wasting time building packages that won't be published.

To resolve this issue, it seems that args.skip_existing should be taken into account to filter the packages present around

pixi/crates/pixi_cli/src/publish.rs

Line 210 in 1a0d4c9

let packages = backend_metadata.metadata.outputs();

cc @pavelzw @wolfv