sync-lockfiles: every dependant keeps its manifest at the toplevel

[milyin]

The crate-path step exists for exactly two repositories — zenoh-java and
zenoh-kotlin, whose Cargo manifest lived under zenoh-jni/. Both are being
rebuilt on the generated zenoh-flat-jni bindings (eclipse-zenoh/zenoh-java#482,
eclipse-zenoh/zenoh-kotlin#669) and no longer contain that crate at all.

They are not leaving this workflow, though. What each keeps is a manifest at the
repository root, whose lockfile records the zenoh-flat-jni commit its CI
builds against:

# zenoh-kotlin/Cargo.toml — compiles to nothing anyone ships
[dependencies]
zenoh-flat-jni = { git = "https://github.com/eclipse-zenoh/zenoh-flat-jni.git", branch = "main" }

Cargo.lock:  source = "git+https://github.com/eclipse-zenoh/zenoh-flat-jni.git?branch=main#<40-hex commit>"

That is the pin their CI reads, and this workflow already knows how to move it:
overwrite the lockfile with zenoh's — which carries no zenoh-flat-jni entry, so
the pin is removed — rectify by resolving again, which writes back the current
commit and compiles it. Same sync, same auto-merging pull request, no special
casing. Rehearsed against the real zenoh lockfile before proposing this: the
rectify step compiled clean in under a minute and the pin came back pointing at
zenoh-flat-jni's main tip.

With no dependant left below the toplevel, the step, its if and the three
interpolations of its output all go. zenoh-c's build-resources/opaque-types
steps are unaffected — they always named their manifest explicitly.

Ordering

Merge after both SDKs have their root manifest on the branch this workflow
syncs, i.e. their default branch:

	root manifest on main?
zenoh-kotlin	eclipse-zenoh/zenoh-kotlin#700, open, CI green
zenoh-java	after eclipse-zenoh/zenoh-java#482 lands, with the same crate

Until then zenoh-java still keeps its manifest under zenoh-jni/, and this would
point its sync at a path that does not exist. Unrelated to #465,
which only adds zenoh-flat and zenoh-flat-jni to the matrix and can merge
whenever.

[milyin]

Folded into #465, which now carries both the new matrix entries and this crate-path removal — one PR for the whole zenoh-java/zenoh-kotlin change to this workflow. Branch drop-crate-path left in place; the commit is cherry-picked onto sync-lockfiles-zenoh-flat unchanged.