[BUG] Fix incorrect Rust library path in `nuttx_add_rust.cmake`

[toku-mac]

Description / Steps to reproduce the issue

Description

When building a Rust crate using CMake with a specific target triple (e.g., thumbv8m.main-nuttx-eabihf), the linker fails to find the generated static library (lib<crate>.a).

This is because nuttx_add_rust.cmake incorrectly constructs the build directory path by appending the target triple to the --target-dir argument. Since Cargo itself also appends the target triple to the directory specified by --target-dir, it results in a nested directory structure like:
.../target/<target-triple>/<target-triple>/debug/lib<crate>.a

However, the CMake script expects the library to be at:
.../target/<target-triple>/debug/lib<crate>.a

Steps to Reproduce

1. Configure NuttX for a board that supports Rust (e.g., nucleo-h563zi:nsh).
2. Use CMake to build the project:

   cmake -S . -B build-debug -DBOARD_CONFIG=nucleo-h563zi:nsh -GNinja
   cmake --build build-debug -t olddefconfig
   cmake --build build-debug -j 6

3. The build fails during the linking stage with an error similar to:

   /path/to/arm-none-eabi/bin/ld: cannot find apps/examples/rust/hello/hello/target/thumbv8m.main-nuttx-eabihf/debug/libhello.a: No such file or directory
   

Expected Behavior

The Rust library should be correctly located by the linker, and the build should complete successfully.

Actual Behavior

The linker fails because it looks for the library in the wrong location due to the nested directory structure created by the incorrect --target-dir usage.

Proposed Fix

Modify apps/cmake/nuttx_add_rust.cmake to set RUST_BUILD_DIR without the target triple and update RUST_LIB_PATH to include the target triple in the expected location.

# Before
set(RUST_BUILD_DIR
    ${CMAKE_CURRENT_BINARY_DIR}/${CRATE_NAME}/target/${TARGET_BASE})
set(RUST_LIB_PATH ${RUST_BUILD_DIR}/${RUST_PROFILE}/lib${CRATE_NAME}.a)

# After
set(RUST_BUILD_DIR
    ${CMAKE_CURRENT_BINARY_DIR}/${CRATE_NAME}/target)
set(RUST_LIB_PATH ${RUST_BUILD_DIR}/${TARGET_BASE}/${RUST_PROFILE}/lib${CRATE_NAME}.a)

On which OS does this issue occur?

[OS: Mac]

What is the version of your OS?

26.4.1

NuttX Version

12.13.0

Issue Architecture

[Arch: arm]

Issue Area

[Area: Examples]

Host information

No response

Verification

• I have verified before submitting the report.

[cederom]

Thank you @toku-mac could you please create fix PR?

@lupyuen is our rust master, and @simbit18 @raiden00pl are our cmake masters, could you please take a look? :-)