fix different behavior between --version and -V options

[alaniwi]

Fix a little bug where mamba --version was displaying the mamba version but mamba -V was not (despite the two options being described as equivalent in the mamba -h output).

For completeness, I've tested it as follows:

• install miniforge (without mamba)
• activate base env
• conda install libmambapy
• python setup.py install
• try mamba -V and mamba --version (and a couple of unrelated options)

Thanks to @jaimergp for identifying which line of code needed to be fixed.

[Hind-M]

Thanks!