GCNV yaml - Mamba >1.2 disallows mixing of conda-forge and defaults channel

[Nicolai-vKuegelgen]

Describe the bug
Since mamba 1.3 their docs mention that mixing of conda-forge and defaults channels is not supported anymore. Snappy currently does this in the enviroment.yaml of the GCNV wrappers.
This file still can be used with mamba 1.2 or conda (any version apparently) but mamba >=1.3 will fail to build an enviroment from it.

To Reproduce
Steps to reproduce the behavior:

1. mamba install mamba>=1.3
2. mamba env create --file snappy_wrappers/wrappers/gcnv/environment.yaml

Expected behavior
Ideally an enviroment.yaml that works with the newest mamba versions should be used.
If this is not possible a different long term solution is needed.

Additional context
Doc changes in Mamba 1.3

[Nicolai-vKuegelgen]

It seems that leaving out the explicit definition of the default ('default::') channel makes the environment.yaml work again with recent conda versions. These packages will still be taken from the de-facto defaults channel (now called pkgs/main), since they are not available elsewhere, so I think this is a working fix (at least until conda changes again).

Will initiate a PR