ci(fuzz): use pre-installed cargo-fuzz to fix OSS-Fuzz build (thiserror E0463)

`cargo install cargo-fuzz --force` rebuilt cargo-fuzz (and its proc-macro dep
`thiserror_impl`) under the base-builder-rust image's sanitizer RUSTFLAGS, which
failed the pr_fuzzing checks with `error[E0463]: can't find crate for thiserror_impl`
before any project code compiled. The image already ships cargo-fuzz, so use it;
only fall back to a clean (RUSTFLAGS-cleared) install if genuinely missing.

Note: not locally reproducible without the OSS-Fuzz container; the pr_fuzzing CI
checks are the verification.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01KfgJznd6jzSeDYsSXGAXkU

Author: claude

.clusterfuzzlite/build.sh

@@ -6,8 +6,16 @@ set -euo pipefail
 
 cd "$SRC"/valence-shell/impl/rust-cli
 
-# Install cargo-fuzz if not present
-cargo install cargo-fuzz --force
+# The OSS-Fuzz base-builder-rust image already ships cargo-fuzz. Reinstalling it
+# with `cargo install --force` rebuilds cargo-fuzz and its dependencies —
+# including the proc-macro crate `thiserror_impl` — under the image's sanitizer
+# RUSTFLAGS, which breaks the host build with:
+#   error[E0463]: can't find crate for `thiserror_impl`
+# So prefer the pre-installed cargo-fuzz, and only fall back to a clean install
+# (RUSTFLAGS cleared) if it is genuinely missing.
+if ! command -v cargo-fuzz >/dev/null 2>&1; then
+    RUSTFLAGS="" cargo install cargo-fuzz --locked
+fi
 
 # Build all fuzz targets
 cargo +nightly fuzz build