libmamba: Tell the build system to use python312

[ryandesign (Ryan Carsten Schmidt)]

libmamba depends on the python312 port but doesn't tell the build system to use that version of python. This may result in the build system using a different python that the user happens to have installed—a python with which the build system might not be compatible. Better to tell the build system which python to use, using a configure argument, environment variable, or whatever other means are provided for that by the build system.