The libsql wrap ran `cargo build` and then copied the archive from
<src>/target/<profile>. Cargo's output directory is not fixed: an ambient
CARGO_TARGET_DIR, CARGO_BUILD_TARGET_DIR or a build.target-dir in a cargo
config relocates it, so the copy step fails *after* a successful compile
with "libsql output not found". Because it is the last step of the build
it reads as a compile failure, and cargo's own success is easy to miss.
Hand the target directory to cargo explicitly as a meson-owned path under
the build tree, and drop CARGO_TARGET_DIR from the child environment.
--target-dir outranks every cargo configuration source, so the directory
cargo writes and the directory we read can no longer disagree, and two
meson build directories no longer share rust artifacts.
Internal only: no user-facing change.
