Set SPIRV_RUST_TARGET_ENV_DEFINE=1 in CMake cargo invocation

When building the Rust FFI library via CMake, we need to set
SPIRV_RUST_TARGET_ENV_DEFINE=1 so that context_bridge.cc compiles
with stub implementations that don't depend on SPIRV-Tools-reduce.

Without this, context_bridge.cc includes full C++ implementations
that reference the Reducer class, but the shared library doesn't
link against SPIRV-Tools-reduce, causing undefined symbol errors.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

Author: LegNeato

CMakeLists.txt

@@ -163,7 +163,9 @@ if(SPIRV_ENABLE_RUST_TARGET_ENV)
 
   add_custom_command(
     OUTPUT ${SPIRV_RUST_LIB_PATH}
-    COMMAND ${CMAKE_COMMAND} -E env SPIRV_TOOLS_FFI_SKIP_CPP_LINK=1
+    COMMAND ${CMAKE_COMMAND} -E env
+            SPIRV_TOOLS_FFI_SKIP_CPP_LINK=1
+            SPIRV_RUST_TARGET_ENV_DEFINE=1
             cargo build --manifest-path ${SPIRV_RUST_MANIFEST} -p spirv-tools-ffi
             ${SPIRV_RUST_CARGO_ARGS}
     WORKING_DIRECTORY ${PROJECT_SOURCE_DIR}