[libomptarget] Fix 'libomptarget' libraries being installed twice (#83624)

Summary:
We use `add_llvm_library` as a shorthand for setting up all the
dependencies and libraries we need for the OpenMP offloading runtime as
they depend on a lot of the LLVM utilities. However, we always
explicitly installed these manually. Behind the scenes the function
would then install it again. This was unnoticed because until now the
destinations matched. Now that we want it to optionally go into the
other directory it is duplicating them. Fix this by stating that this is
a build tree only library so we can handle it ourselves.