fix: Propagate target-platform flags for pixi build

[AdamDorwart]

During some experimentation with pixi build --target-platform <target> I noticed if I had multiple packages in a workspace with dependencies they would build fine with just pixi build but adding --target-platform would fail. If it was just a single independent package it worked fine but the dependencies would report as not existing for the target platform.

After a bit of digging I discovered this fix would propagate the target platform correctly so that transitive dependencies would build for my target platform.

[AdamDorwart]

I'm still testing to make sure this works the way I expect but wanted to open the PR to start the conversation and get feedback.

[AdamDorwart]

@ruben-arts I was able to test this locally and confirm it solves the cross compilation issue I was seeing with multiple source packages in a workspace. What else do you need from me to get this merged?

[ruben-arts]

Thank you @AdamDorwart!

This code is mostly untouched by me, @baszalmstra would you know a way we could confidently write a test for this?

[baszalmstra]

I dont understand, the changes seem to be completely unrelated to the description? Did you accidentally override the changes?

[AdamDorwart]

@baszalmstra my apologies some unrelated changes were merged to this same branch. I will fix this up shortly.

Only the first commit with the one line change was intended for this PR.

[baszalmstra]

Ah that makes sense! Your first commit looks good indeed! If you remove the other changes we can merge.

[AdamDorwart]

@baszalmstra Sorry again for that confusion. Should be good to go now.

[baszalmstra]

No worries! Thanks for the contribution!