Remove duplicate `lib/` from install destinations

[fredemmott]

refs #249

Packages currently have a lib/lib subpath in them; as grep shows no lib/lib in this repo and other entries in this file do not have the final component, I'm assuming this is correct. Will test before landing.

[fredemmott]

Manually built, got:

root@8b7e8581420f:CMake# pwd
/opt/hhvm/2020.11.30/lib/x86_64-linux-gnu/hhvm/CMake
root@8b7e8581420f:CMake# ls
EXTFunctions.cmake          FindLibCh.cmake          FindLibZip.cmake        FindlibWatchmanClient.cmake
FindBISON.cmake             FindLibDL.cmake          FindLibiconv.cmake      HHVMExtensionConfig.cmake
FindCClient.cmake           FindLibDwarf.cmake       FindLibinotify.cmake    HHVMGenerateConfig.cmake
FindDoubleConversion.cmake  FindLibElf.cmake         FindLibjsonc.cmake      HHVMProject.cmake
FindEditline.cmake          FindLibEvent.cmake       FindLibmemcached.cmake  HPHPCompiler.cmake
FindFLEX.cmake              FindLibGmp.cmake         FindLibpam.cmake        HPHPFindLibs.cmake
FindFastLZ.cmake            FindLibIntl.cmake        FindMcrypt.cmake        HPHPFunctions.cmake
FindFreetype.cmake          FindLibJpeg.cmake        FindONIGURUMA.cmake     HPHPIZEFunctions.cmake
FindGLIB.cmake              FindLibMagickWand.cmake  FindPCRE.cmake          HPHPSetup.cmake
FindGlog.cmake              FindLibNuma.cmake        FindPThread.cmake       MSVCDefaults.cmake
FindGold.cmake              FindLibPng.cmake         FindRE2.cmake           Options.cmake
FindICU.cmake               FindLibSQLite.cmake      FindReadline.cmake      ThinArchives.cmake
FindLZ4.cmake               FindLibSodium.cmake      FindSnappy.cmake        VisualStudioToolset.cmake
FindLdap.cmake              FindLibVpx.cmake         FindTBB.cmake           cotire.cmake
FindLibAfdt.cmake           FindLibXed.cmake         Findfribidi.cmake