Explicitly set the location of cpanp-run-perl when under PERL_CORE

Explicitly set the location of cpanp-run-perl when under PERL_CORE

    CPANPLUS::Config checks 3 specific scenarios first
    when looking for cpanp-run-perl: parallel to cpanp,
    parallel to CPANPLUS.pm, or installed into a custom
    prefix like /tmp/foo. Only *THEN* does it check the
    the path.
    If the perl core is extracted to a directory that has
    cpanp-run-perl installed the same amount of 'uplevels'
    as the /tmp/foo prefix, we'll pull in the wrong script
    by accident.
    Since we set the path to cpanp-run-perl explicitily
    at the top of this script, it's best to update the config
    ourselves with a path lookup, rather than rely on its
    heuristics. Thanks to David Wheeler, Josh Jore and Vincent
    Pit for helping to track this down.

Effects both CPANPLUS and CPANPLUS-Dist-Build