enh: Add support for spm/mcr config via env vars

This patch adds support for enforcing and configuring the spm MCR standalone
version via environment variables. The respected environment variables are:

    FORCE_SPMMCR: any value, sets use_mcr to True
    SPMMCRCMD:    command to use when running spm MCR, if use_mcr is True
Fix inconsistent indentation
Catch only KeyErrors when checking for env var