refactor: remove ipc override from options and give it manually to test

[wolfv]

If I understand correctly, creating the IPC backend from a "string" will not be easily possible, and the types used are not serializable or cloneable.

I envision a future were we can override build backends selectively, and so it would be good to be able to use a hash map such as:

pixi-build-rattler-build: /foo/bar/pixi-build-rattler-build-test
bla: ...

But to achieve this, we need a type that is easier to use than the current Io backend. Since that backend override is only used in testing, we can use more direct methods of setting it in the Protocol.

Lmk what you think @tdejager / @baszalmstra

[wolfv]

Nice work!