fix: ensure sourceMappingUrl is correct

[adamstankiewicz]

Description

Ensures sourceMappingUrl in generated *.min.css is relative to the same directory such that it can be loaded when requested.

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[edx-semantic-release]

🎉 This PR is included in version 21.0.0-alpha.35 🎉

The release is available on:

• npm package (@alpha dist-tag)
• GitHub release

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[edx-semantic-release]

🎉 This PR is included in version 22.0.0-alpha.1 🎉

The release is available on:

• npm package (@alpha dist-tag)
• GitHub release

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[openedx-semantic-release-bot]

🎉 This PR is included in version 23.0.0-alpha.1 🎉

The release is available on:

• npm package (@alpha dist-tag)
• GitHub release

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