In most cases, the RPATH is stripped out when the ELF file is packages,
but by then the damage is done from a reproducible perspective because
this absolute path is hashed as part of the build-id generated at link
time ([1] has a good explanation). Fortunately, newer cmake has an
option to generated relative RPATHs that use $ORIGIN to set the path, so
set it in the toolchain file.
