Verify that backend environment solving picks up the mirror config

[Hofer-Julian]

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[Hofer-Julian]

This is quite interesting so far.

As one can see in the failing test, pixi build doesn't pick up the mirror configuration.

However, there is yet another layer to it. If there's a mirrored channel in the workspace channels, that will be picked up, even if that channel is overridden in the package section. This can be seen here: Hofer-Julian#1

Channel overriding in general works, I tested it with a simple example without mirrors involved.
Mirrors also work for the normal workspace use case.
It's not a regression, the test also fails with the latest releases.