Allow env vars for RUN_DISTUTILS
Allow env vars for RUN_DISTUTILS

Allow use of env vars RUN_DISTUTILS, RUN_DISTUTILOPTS as defaults.

With this, on macos, macports doesn't need to be in PATH to build FAHControl. One just needs

export RUN_DISTUTILS="/opt/local/bin/python"

or the equivalent in dockbot.json env.

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