[test][compiler-rt] Avoid LD_PRELOAD for "outer" dynamic linkers

Summary:
This patch moves the setting of `LD_PRELOAD` "inwards" to avoid issues
where the built library needs to be loaded with the dynamic linker that
was configured with the build (and cannot, for example, be loaded by the
dynamic linker associated with the `env` utility).

Reviewed By: vitalybuka, nemanjai, jsji

Differential Revision: https://reviews.llvm.org/D79695