fix: url() with variable or relative path in sass/scss is broken (fixes #7651)

[frankyeyq]

Description

fixes #7651

Additional context

---

What is the purpose of this pull request?

• [x] Bug fix
• [ ] New Feature
• [ ] Documentation update
• [ ] Other

Before submitting the PR, please make sure you do the following

• [x] Read the Contributing Guidelines.
• [x] Read the Pull Request Guidelines and follow the Commit Convention.
• [x] Check that there isn't already a PR that solves the problem the same way to avoid creating a duplicate.
• [x] Provide a description in this PR that addresses what the PR is solving, or reference the issue that it solves (e.g. fixes #123).
• [ ] Ideally, include relevant tests that fail without this PR but pass with it.

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[makroxyz]

Any news on this PR?

[andyexeter]

Is there anything I can do to help get this PR merged?

I'd love to migrate our projects over from Webpack to Vite, but this is a real blocker for us.

[bluwy]

We need tests for this. Feel free to send another PR that supersedes this with tests!

[ioan-cristea]

Any news?

[andyexeter]

I tried implementing this into a new PR with tests but a load of other seemingly unrelated tests started failing because of it.

This is out of my area of expertise so not something I'm going to be able to help with unfortunately.