fix(ci): test-doc must not persist to /cache (was filling 27G per run)

test-doc has SKIP_WASM_BUILD=0 which compiles WASM runtimes and doc tests,
producing 27G+ of artifacts. It was persisting those to /cache/cargo-target/
alongside the test-release dir, filling the 96G kurditv volume to 100%.

Since test-doc is allow_failure and the warm-cache benefit doesn't justify
the disk cost, switch it to the ephemeral container filesystem (same approach
as clippy). The before_script no longer sets CARGO_TARGET_DIR.

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

.gitlab-ci.yml

@@ -557,16 +557,14 @@ test-doc:
   variables:
     CARGO_HOME: /root/.cargo
     RUSTFLAGS: "-Cdebug-assertions=y -Dwarnings"
+    # SKIP_WASM_BUILD=0: doc tests need actual runtime types. With WASM build
+    # enabled, artifacts can reach 27G+ — intentionally NOT persisted to /cache
+    # (allow_failure job; no warm-cache benefit justifies the disk cost).
     SKIP_WASM_BUILD: "0"
   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}/${CI_JOB_NAME}"
-    - mkdir -p "$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"
   cache: []
   timeout: 5h
   retry: