Allow disabling strict channel priority in `pixi global install ...`

Problem description

Hi,

We publish packages to an internal artifactory server that use a mixture of internal and conda-forge dependencies. However, this layering is inconsistent with strict channel priorities. Whilst one can get around this in pixi projects (https://github.com/prefix-dev/pixi/discussions/1029), but I don't believe that is compatible with pixi global install ....

So the feature request is to enable mixing channels in pixi global install ..., but appreciate it might not be as simple as that.

Many thanks!