[legacy_promotions] Don't persist line item actions in compute_amount

[forkata]

Summary

This change will be used in the new in memory order updater as part of #5872.

We only want to persist to the database once we perform the action, not when we
are computing the amount.

Checklist

Check out our PR guidelines for more details.

The following are mandatory for all PRs:

• I agree that my PR will be published under the same license as Solidus.
• I have written a thorough PR description.
• I have kept my commits small and atomic.
• I have localized any and all user-facing strings that I added to the source code.
• I have used clear, explanatory commit messages.

The following are not always needed:

• 📖 I have updated the README to account for my changes.
• 📑 I have documented new code with YARD.
• 🛣️ I have opened a PR to update the guides.
• ✅ I have added automated tests to cover my changes.
• 📸 I have attached screenshots to demo visual changes.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.46%. Comparing base (57e8ef9) to head (dc32b3b).
⚠️ Report is 8 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #6393   +/-   ##
=======================================
  Coverage   89.46%   89.46%           
=======================================
  Files         977      977           
  Lines       20386    20389    +3     
=======================================
+ Hits        18238    18241    +3     
  Misses       2148     2148           

☔ 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.