Skip cirq pre-release if identical to the last version

[pavoljuhas]

1. Improve the produce-package.sh script for wheel building

• add -h, --help option to show usage information
• add -c, --commit=COMMIT option for building from sources
  at COMMIT instead of HEAD
• respect SOURCE_DATE_EPOCH if available as environment variable.
  Otherwise get it from the commit time of the sources.
• do not purge temporary files if script exits with an error

2. Update the release-main workflow to skip release if identical to the last one

[pavoljuhas]

Here are examples of the pre-release workflow when the distribution content is:

• identical - https://github.com/quantumlib/Cirq/actions/runs/19085889607
• changed - https://github.com/quantumlib/Cirq/actions/runs/19085979383

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.38%. Comparing base (21ee064) to head (098bca4).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #7737   +/-   ##
=======================================
  Coverage   99.38%   99.38%           
=======================================
  Files        1091     1091           
  Lines       97912    97912           
=======================================
  Hits        97306    97306           
  Misses        606      606           

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.

[mhucka]

A couple of nits, at your discretion to do or leave.