Provide for passing credentials to SRPMfromSCM
    
    Some places force every access to their systems to be authenticated for
    audit purposes, which means even an https fetch of a source tarball
    needs access to some sort of credential.
    
    Provide for a directory intended to contain credentials to be
    bind-mounted into the BuildRoot for the BuildSRPMFromSCM task as
    "/credentials". Whatever runs a a result of the source_cmd ("make
    sources") is responsible for doing something useful with the contents.