removed absolute path for valgrind and gcov, now relies on users environment
removed absolute path for valgrind and gcov, now relies on users environment

johnnyliu27/openmc,sxds/opemmc,wbinventor/openmc,shikhar413/openmc,walshjon/openmc,paulromano/openmc,amandalund/openmc,shenqicang/openmc,mit-crpg/openmc,shikhar413/openmc,wbinventor/openmc,bhermanmit/openmc,lilulu/openmc,shenqicang/openmc,liangjg/openmc,liangjg/openmc,shikhar413/openmc,liangjg/openmc,nhorelik/openmc,walshjon/openmc,johnnyliu27/openmc,samuelshaner/openmc,mit-crpg/openmc,paulromano/openmc,smharper/openmc,mit-crpg/openmc,amandalund/openmc,shikhar413/openmc,mjlong/openmc,sxds/opemmc,walshjon/openmc,johnnyliu27/openmc,mjlong/openmc,amandalund/openmc,samuelshaner/openmc,smharper/openmc,lilulu/openmc,wbinventor/openmc,mit-crpg/openmc,walshjon/openmc,samuelshaner/openmc,nhorelik/openmc,smharper/openmc,kellyrowland/openmc,wbinventor/openmc,johnnyliu27/openmc,samuelshaner/openmc,amandalund/openmc,paulromano/openmc,paulromano/openmc,lilulu/openmc,bhermanmit/openmc,smharper/openmc,liangjg/openmc,kellyrowland/openmc