[lit] Only set DYLD_LIBRARY_PATH for shared builds

In r370135 I committed a temporary workaround for the sanitized bot to
not set (DY)LD_LIBRARY_PATH when (DY)LD_INSERT_LIBRARIES was set.
Setting (DY)LD_LIBRARY_PATH is only necessary for (standalone)
shared-library builds, so a better solution is to only set the
environment variable when necessary.

Differential revision: https://reviews.llvm.org/D67012

llvm-svn: 370549