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.