[lldb/CMake] Use INSTALL_RPATH for tools and BUILD_RPATH for unittests.

It seems like only the unittests are building with
BUILD_WITH_INSTALL_RPATH set to OFF. Of course when I did my last change
I only ran check-lldb-unit. Not sure why this difference exists, why
would you even install the unittest?

For the LLDB framework we do need different build and install RPATHs.
Currently that logic lives downstream. I plan to upstream that in the
near future. For now I'm just trying to make it possible to run the
test.