Fix uncontrolled cast of size_t to uint

[j-rivero]

🦟 Bug fix

Remove warning existing in Conda Windows builds. See https://build.osrfoundation.org/view/conda/job/gz_sim-ign-gazebo6-cwin/1/msbuild/

Summary

Fix the warning:

'argument': conversion from 'size_t' to 'unsigned int', possible loss of data (compiling source file C:\J\workspace\gz_sim-ign-gazebo6-cwin\ws\gz-sim\src\SimulationRunner.cc)

by controlling the cast from site_t to unsigned int.

Fixed here:

Checklist

• Signed all commits for DCO
• Added tests
• Updated documentation (as needed)
• Updated migration guide (as needed)
• Consider updating Python bindings (if the library has them)
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• All tests passed (See test coverage)
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