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>
Merge pull request #843 from davidlt/patchelf-use-cms-install-prefix

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