Protect against CSH_PATH being set already

Following on from INC3305970 and

https://github.com/cms-sw/cmssw/issues/40013#issuecomment-1324032588

some CMS tcsh users want to set CMS_PATH to a custom path.

As things stands CMS_PATH was being clobbered by hepix for every shell.

This has the added benefit that /cvmfs/cms.cern.ch will not be probed on every tcsh invocation.