rust: support linking a prebuilt install via TRANSCRIBE_DIR (#117)

Setting TRANSCRIBE_DIR (OPENSSL_DIR-style) to an install prefix produced
by cmake --install of a TRANSCRIBE_INSTALL=ON build skips the vendored
source build entirely: the link line is emitted from that prefix's
lib*/transcribe-link.json through the existing find_manifest /
emit_link_lines path, so static and shared prefixes both work with no
new link logic. Cargo features are inert on this path (the prebuilt
already fixed its configuration; the manifest records it). Unset, the
source build is byte-identical to before.