scram-project-build: use $CMS_INSTALL_PREFIX for glibc path

`$RPM_INSTALL_PREFIX` is not the final destination for CMSSW, we
might install _CMSSW_ locally and then move to AFS. `$CMS_INSTALL_PREFIX`
holds the final destination for software.

Signed-off-by: David Abdurachmanov <David.Abdurachmanov@cern.ch>