Meta: Use correct variable for checking if the mold linker is used

This variable was originally called USE_MOLD_LINKER, but it was changed
to ENABLE_MOLD_LINKER during review to be consistent with other
configuration options. I branched off the commits that added RELR
support before this change, and I failed to update the variable name
there.

CMakeLists.txt

@@ -280,7 +280,7 @@ if(NOT "${SERENITY_ARCH}" STREQUAL "aarch64")
  add_link_options(-fuse-ld=mold)
  endif()
 
- if (CMAKE_CXX_COMPILER_ID MATCHES "Clang$" OR USE_MOLD_LINKER)
+ if (CMAKE_CXX_COMPILER_ID MATCHES "Clang$" OR ENABLE_MOLD_LINKER)
  add_link_options(LINKER:--pack-dyn-relocs=relr)
  else()
  add_link_options(LINKER:-z,pack-relative-relocs)