CPack README.txt Handling Fix, master branch (2020.01.06)

Back when I updated the (normal) RPM builds to put the "configured" REAMDE.txt file in the CMakeFiles/ directory, I forgot to update the "opt-dbg" package configuration file. 😦 Resulting in some CPack error messages in the nightlies that went unnoticed for a long time. Like:

CPack: Create package using RPM
CPack: Install projects
CPack: - Install scripts: /build/atnight/localbuilds/nightlies/master/build/install/AthenaExternals/22.0.9/InstallArea/x86_64-centos7-gcc8-opt/cmake/modules/scripts/cpack_install.cmake
CPack: - Install script: /build/atnight/localbuilds/nightlies/master/build/install/AthenaExternals/22.0.9/InstallArea/x86_64-centos7-gcc8-opt/cmake/modules/scripts/cpack_install.cmake
CPack: Create package
-- CPackRPM:Debug: Using CPACK_RPM_ROOTDIR=/build/atnight/localbuilds/nightlies/master/build/build/Athena/_CPack_Packages/Linux/RPM
CPackRPM: Will use GENERATED spec file: /build/atnight/localbuilds/nightlies/master/build/build/Athena/_CPack_Packages/Linux/RPM/SPECS/Athena_22.0.9_x86_64-centos7-gcc8-opt.spec
CPack: - package: /build/atnight/localbuilds/nightlies/master/build/build/Athena/Athena_22.0.9_x86_64-centos7-gcc8-opt.rpm generated.
time::time -p  "cpack":: real 1593.48
user 1165.63
sys 213.86
CPack: Create package using RPM
CPack Error: Cannot find description file name: [/build/atnight/localbuilds/nightlies/master/build/build/Athena/README.txt]
CPack Error: Error when generating package: Athena
time::time -p  "cpack" "--config" "CPackDbgRPMConfig.cmake":: real 0.29
user 0.01
sys 0.02

This was finally pointed out to me in ATLINFR-3333.