fix aarch64 wheel build: keep make's link target in place; arch-aware libfaust path

The CI before_build step moved build/libdawdreamer.so away, so setup.py's
make (run again by python -m build) always relinked. That relink invoked
make without the aarch64 libfaust -L flag, and the Projucer-generated
Makefile hardcodes the ubuntu-x86_64 directory, so aarch64 failed with
"cannot find -lfaustwithllvm" (x86_64 was silently rebuilding twice).

- CI and the Dockerfile now cp instead of mv, so setup.py sees an
  up-to-date build and skips recompiling entirely.
- setup.py passes LDFLAGS with the ubuntu-$(machine) libfaust lib
  directory so that a rebuild also works on aarch64, including local
  builds on ARM machines.