Pixi: move deps to run-dependencies section

[VeckoTheGecko]

When defining Parcels as a source dependency, e.g.,

[dependencies]
virtualship = { path = "." }
parcels = {path="./Parcels"}
# ...

you need to define the run dependencies in package.run-dependencies not in [dependencies] (which is workspace dependencies). Otherwise the dependencies aren't properly installed.

This makes it much clearer how installing Parcels as a development dependency works (allowing benchmarking, and people define manifests in their scientific project that use an unreleased version of Parcels).

xref: discord

[VeckoTheGecko]

CI passed! 🥳