Use commit timestamps for plugin EP dev versions (#32095)
### Description
<!-- Describe your changes. -->
- Use commit timestamps for plugin EP dev versions
- Update set_plugin_ep_build_variables.py to use argparse
### Motivation and Context
<!-- - Why is this change required? What problem does it solve?
- If it fixes an open issue, please link to the issue here. -->
Python dev package versions were using only the commit date. This
prevented packages from multiple commits on the same day from being
published. Using a finer granularity (from day to seconds) should
mitigate it. It's probably good enough if we only expect to publish from
`main` and release branches.
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Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>
