Fix weekly builds: replace msbuild with cmake --build

[Copilot]

Windows weekly builds fail with MSB1009: Project file does not exist because windows-latest now defaults to the VS 18 (2026) generator, and calling msbuild with a hardcoded .sln filename is fragile against generator changes.

Changes

• Replace msbuild invocations with cmake --build across all three Windows build steps (ZXSpectrum, C64, CPC):

  - msbuild /m /p:Configuration=${{ env.BUILD_CONFIGURATION }} SpectrumAnalyser.sln
  + cmake --build . --config ${{ env.BUILD_CONFIGURATION }}

  cmake --build delegates to the correct underlying build tool without needing to know the solution filename.

• Update microsoft/setup-msbuild from v1.0.2 → v2 to eliminate the deprecated set-output warning.