Relative channels should be relative to pixi.toml

[wolfv]

Checks

• [X] I have checked that this issue has not already been reported.

• [X] I have confirmed this bug exists on the latest version of pixi, using pixi --version.

Reproducible example

channels = ["../output"]

Issue description

When using a relative channel (e.g. ../output) resolving from a different folder than the pixi root folder stops working.

Expected behavior

We should compute the absolute path as something like $PIXI_ROOT.join(channel_dir).

[wolfv]

Started to work on this in https://github.com/mamba-org/rattler/pull/602

[wolfv]

Not fixed yet, unfortunately.