fix: don't check for PMUs in use if noroot flag given

[harp-intel]

This pull request updates the logic in the prepareTarget function in cmd/metrics/metrics.go to ensure that certain configuration steps are only performed when root access is available. The changes improve the robustness of target configuration by properly gating PMU-related checks and actions behind the flagNoRoot condition.

Target configuration logic improvements:

• Wrapped the PMU usage check and related script execution in a conditional block that only runs if root access is available (!flagNoRoot), preventing unnecessary or failing operations on targets without root access.
• Ensured that the conditional block for disabling the NMI watchdog is properly separated from the PMU check, maintaining clear and correct flow for root-dependent actions.