Revert "[CMake] Don't enable BUILD_WITH_INSTALL_RPATH when using custom build rpath"

This reverts commit 0ebc1fb2.

The behaviour should have been the same as before unless specifying CMAKE_BUILD_RPATH,
which was previously broken.
However, this seems to have broken builds for some people that don't specify it.
Reverting until I can investigate.

Differential Revision: https://reviews.llvm.org/D94319