fix CMS instructions

[jpata]

Based on the discussion with Andy, the RelVals for the matrix workflows are no longer available (strange).
However, we don't really need the matrix workflows for testing in the first place, it's better and more direct to test directly on GEN-SIM-DIGI-RECO (a la PFValidation).

----- Begin Fatal Exception 08-Jul-2024 23:07:06 CEST-----------------------
An exception of category 'FallbackFileOpenError' occurred while
   [0] Constructing the EventProcessor
   [1] Constructing module: class=MixingModule label='mix'
   [2] Calling RootInputFileSequence::initTheFile()
   [3] Calling StorageFactory::open()
   [4] Calling XrdFile::open()
Exception Message:
Failed to open the file 'root://xrootd-cms.infn.it//store/relval/CMSSW_13_1_0_pre1/RelValMinBias_14TeV/GEN-SIM/130X_mcRun3_2022_realistic_withNewBSFromEOY2022Data_v2_RV186-v1/00000/3a417d3c-eada-4641-99e8-31c73d1cdbec.root'
   Additional Info:
      [a] Calling RootInputFileSequence::initTheFile(): fail to open the file with name root://eoscms.cern.ch//eos/cms/store/relval/CMSSW_13_1_0_pre1/RelValMinBias_14TeV/GEN-SIM/130X_mcRun3_2022_realistic_withNewBSFromEOY2022Data_v2_RV186-v1/00000/3a417d3c-eada-4641-99e8-31c73d1cdbec.root
      [b] Input file root://xrootd-cms.infn.it//store/relval/CMSSW_13_1_0_pre1/RelValMinBias_14TeV/GEN-SIM/130X_mcRun3_2022_realistic_withNewBSFromEOY2022Data_v2_RV186-v1/00000/3a417d3c-eada-4641-99e8-31c73d1cdbec.root could not be opened.
      [c] XrdCl::File::Open(name='root://xrootd-cms.infn.it//store/relval/CMSSW_13_1_0_pre1/RelValMinBias_14TeV/GEN-SIM/130X_mcRun3_2022_realistic_withNewBSFromEOY2022Data_v2_RV186-v1/00000/3a417d3c-eada-4641-99e8-31c73d1cdbec.root', flags=0x10, permissions=0660) => error '[ERROR] Server responded with an error: [3011] No servers are available to read the file.
' (errno=3011, code=400). No additional data servers were found.
      [d] Last URL tried: root://cms-xrd-global.cern.ch:1094//store/relval/CMSSW_13_1_0_pre1/RelValMinBias_14TeV/GEN-SIM/130X_mcRun3_2022_realistic_withNewBSFromEOY2022Data_v2_RV186-v1/00000/3a417d3c-eada-4641-99e8-31c73d1cdbec.root?tried=+1213llrxrd-redir.in2p3.fr,&xrdcl.requuid=57004fda-81c5-4b19-a712-48ff4ac9956d
      [e] Problematic data server: cms-xrd-global.cern.ch:1094
      [f] Disabled source: cms-xrd-global.cern.ch:1094
----- End Fatal Exception -------------------------------------------------

Also fix env vars in the validation script.