[Build] Fix Windows ARM64 CUDA plugin build

[tianleiwu]

The Windows ARM64 CUDA plugin packaging job passes --skip_tests, but that option skips test execution only; it does not disable generation and compilation of unit-test targets. Because onnxruntime_BUILD_UNIT_TESTS defaults to ON, the job still builds onnxruntime_mlas_test.

In the failing CI run, the MLAS library itself compiled and linked successfully, including sqnbitgemm_kernel_avx512_2bit.cpp. The later failure was an ARM64 MSVC compiler crash while compiling the unused onnxruntime_mlas_test target: CL.exe exited with code 57005 (0xDEAD) during test_sqnbitgemm_2bit.cpp.

This change sets onnxruntime_BUILD_UNIT_TESTS=OFF only for the Windows ARM64 CUDA plugin packaging job. The CUDA plugin and MLAS library are still built, and the existing plugin binary verification remains enabled; only unit-test targets that are not run by this packaging job are excluded.

Note: A more complete fix is to split sqnbitgemm_kernel_avx512_2bit.cpp into multiple files.

Validation:

• YAML parsed successfully.
• Confirmed the define is present only in the ARM64 CUDA plugin build block.
• git diff --check passed.
• Windows ARM64 CI build remains to be verified by this PR.