[ENH] Micromamba installer

[ales-erjavec]

Issue

Newer Microconda installers no longer register their location in the Windows registry so miniconda install location cannot be reliably determined in the installer.

Changes

• Use a static fully contained micromamba executable to create the target environment. This is fully self contained and does not depend on Miniconda/Anaconda.
• Use mamba instead of conda to manage the environment (add-on dialog)
• Use gh-actions to build the installers.