Need explicit warning message for upcoming lack of support for nodefaults

[trallard]

🎉 I just noticed the addition of #367 (yay).

Though, the warning message shown (about the usage of nodefaults to remove defaults) in the GitHub actions summary can be improved to make it explicit that this is coming from setup-miniconda (see screenshot for the current warning message).

Something like `'nodefaults' channel detected: will remove 'defaults' if added implicitly. In the future, 'nodefaults' to remove 'defaults' won't be supported. Please set 'conda-remove-defaults' = 'true' in setup-miniconda to remove this warning.