fix(ci): add WASM_BUILD_WORKSPACE_HINT to test-doc job

Without this env var, wasm-builder cannot find Cargo.lock when
CARGO_TARGET_DIR is outside the workspace (/cache/target). This causes
the nested wasm build to resolve fresh dependencies, pulling alloy-eips
1.7.3 which requires rustc 1.91 (CI has 1.88.0).

.github/workflows/docs.yml

@@ -31,6 +31,7 @@ jobs:
       RUSTFLAGS: "-Cdebug-assertions=y -Dwarnings"
       CARGO_INCREMENTAL: 0
       CARGO_TARGET_DIR: /cache/target
+      WASM_BUILD_WORKSPACE_HINT: ${{ github.workspace }}
     steps:
       - uses: actions/checkout@08c6903cd8c0fde910a37f88322edcfb5dd907a8 # v5.0.0