Refactor duplicate CPU time calculation

[VolodymyrBg]

Describe your changes.

Extract duplicated time calculation logic in benchmarks/src/utils.rs into a reusable helper function.

Created a new helper function calculate_cpu_time to eliminate code duplication when calculating
CPU time from rusage structs. This function is now used in the phase_end function to calculate
both initial and final CPU times, making the code more maintainable and easier to understand.

This refactoring:

• Reduces code duplication
• Improves readability
• Makes the code more maintainable
• Preserves the exact same functionality

[github-actions]

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[VolodymyrBg]

I have read the CLA Document and I hereby sign the CLA

[sjudson]

Thanks a lot for the PR. Ultimately this is a minor aesthetic change in some benchmarking infrastructure, and as it doesn't fix a concrete issue or contribute new functionality, I'm going to pass on it.