Save solidus_admin artifacts during Actions workflows

[benjaminwil]

Summary

It looks like Circle CI had been configured to do this based on the presence of the CIRCLE_ARTIFACTS environment variable. But I found no trace of artifact saving in the GitHub Actions workflow configuration.

I would like to view solidus_admin artifacts if feature tests fail on CI. This commit adds the necessary configuration, in the same style that we enable code coverage reporting per workflow (with a boolean input setting).

Verification

I manually tested that this configuration commit is valid on another pull request, #6610.

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 92.30%. Comparing base (02f8479) to head (1fc5c39).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #6611   +/-   ##
=======================================
  Coverage   92.30%   92.30%           
=======================================
  Files        1047     1047           
  Lines       21336    21336           
=======================================
  Hits        19694    19694           
  Misses       1642     1642           

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