Follow up to 35dd959133fb233d75e9b3cddbf98b2ed01c6594, allows explicitly
adding the unix-ffi library path from the command line.
This option is needed when building unix-ffi manifests in micropython-lib
CI.
This work was funded through GitHub Sponsors.
Signed-off-by: Angus Gratton <angus@redyak.com.au>