[dotest] Set environment variables after potentialy clearing others.

Dotest contains code to clear DYLD_LIBRARY_PATH for the inferior to not
propagate sanitized builds. However, it's possible that we want to
inject a different library path with `--inferior-env`. To make that work
correctly, we need to do that *after* clearing DYLD_LIBRARY_PATH.

llvm-svn: 367054