Use main channel instead of defaults

[antonwolfy]

It's proper to explicitly define main channel when installing packages from conda rather than to rely on a value of defaults channel in conda configuration.
This is because main channel is what exactly required for building and testing dpnp and also because conda is configured by conda-incubator/setup-miniconda github action and we have no control on that (can't guarantee defaults channel is the same as main one).

• Have you provided a meaningful PR description?
• Have you added a test, reproducer or referred to issue with a reproducer?
• Have you tested your changes locally for CPU and GPU devices?
• Have you made sure that new changes do not introduce compiler warnings?
• If this PR is a work in progress, are you filing the PR as a draft?