Skip test builds in the non-runnable ARM64 mwin32 SDK job

[Copilot]

The mwin32 SDK ARM64 Actions job was failing in the build phase because it cross-compiled test executables and then hit GoogleTest discovery, which attempts to execute them on the x64 runner. This change keeps the x64 SDK build/test behavior intact while preventing the ARM64 artifact job from building non-runnable tests.

• Workflow matrix

  • Add a per-arch build_testing setting to the mwin32-sdk.yml matrix.
  • Keep BUILD_TESTING enabled for x64.
  • Disable BUILD_TESTING for arm64, which already has can_run_tests: false.

• CMake configuration

  • Replace the unconditional test build flag with the matrix-driven value during configure.
  • This removes the mismatch where the workflow skipped the test step for ARM64 but still asked CMake to build and discover tests.

• Resulting behavior

  • x64: unchanged; builds with tests and runs the in-scope mwin32 suite.
  • arm64: builds only the SDK artifacts needed for packaging; no test discovery is triggered during build.

matrix:
  include:
    - arch: x64
      can_run_tests: true
      build_testing: "ON"
    - arch: arm64
      can_run_tests: false
      build_testing: "OFF"

# ...
-D BUILD_TESTING=${{ matrix.build_testing }}