ci(clippy): revert persistent CARGO_TARGET_DIR, keep CARGO_HOME fix

librocksdb-sys debug build writes a multi-GB librocksdb.a static archive
to /cache/cargo-target, filling the volume and causing build failure.
Persistent cache is not safe for C++ heavy dependencies.

Keep CARGO_HOME=/root/.cargo (avoids re-downloading the Rust crate
registry on every run). CARGO_TARGET_DIR stays ephemeral (Docker
container fs has ample space).

Timeout stays at 3h to cover cold builds on slower runners.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

.gitlab-ci.yml

@@ -171,18 +171,11 @@ clippy:
     SKIP_WASM_BUILD: "1"
     # Use pre-baked registry in the CI image instead of re-downloading all
     # crates on every run (global CARGO_HOME=$CI_PROJECT_DIR/.cargo is wiped).
+    # NOTE: We intentionally do NOT persist CARGO_TARGET_DIR to /cache here —
+    # librocksdb-sys debug build produces a multi-GB librocksdb.a that fills
+    # the /cache volume. Docker container's ephemeral target dir is safer.
     CARGO_HOME: /root/.cargo
   timeout: 3h
-  before_script:
-    - rustup show
-    - rustc --version && cargo --version
-    - rustup target add wasm32v1-none
-    - export CACHE_KEY="$(cat Cargo.lock rust-toolchain.toml | md5sum | cut -c1-12)"
-    - export CARGO_TARGET_DIR="/cache/cargo-target/${CACHE_KEY}/clippy"
-    - mkdir -p "$CARGO_TARGET_DIR"
-    - find /cache/cargo-target -mindepth 1 -maxdepth 1 -type d ! -name "${CACHE_KEY}" -exec rm -rf {} + 2>/dev/null || true
-    - echo "CARGO_TARGET_DIR=$CARGO_TARGET_DIR (disk $(df -h /cache/cargo-target | tail -1 | awk '{print $3 "/" $2 " used, " $4 " free"}'))"
-  cache: []
   script:
     - git config --global --add safe.directory '*'
     - |