When building custom miniforge installers the user or process may not have access to public repositories due to security restrictions in place on their internal networks. The build scripts can be updated to use their download URL, but then it would require us to update the scripts after we downloaded the miniforge release zip file. We would like to preserve the files as much as possible so it could be beneficial to allow the user to supply a custom location for the micromamba installer. If a custom location is not provided, then fallback to the download from https://anaconda.org/conda-forge/micromamba.