[conda] gcc7-based toolchain goes main

[looo]

There is a big change coming to conda-forge. All gcc-7 labeld builds will be merged into main. This means FreeCAD (most-likely) won't work anymore with the traditional way to install conda-packages. But this doesn't mean we have to hurry with the gcc7 [1] update. Backward-compatibility is managed by adding a roll-back channel. So in theory all we need to do is removing to rename conda-forge into something like "conda-forge/label/cf1901" [2].

[1] https://forum.freecadweb.org/viewtopic.php?f=4&t=31994
[2] https://conda-forge.org/status/

btw.: Most likely there is no change for windows users.