Fix sprite can't be written in relative path("sprite": "css/sprite").

[zjp369]

Launch Checklist

• briefly describe the changes in this PR
• include before/after visuals or gifs if this PR includes visual changes
• write tests for all new functionality
• document any changes to public APIs
• post benchmark scores
• manually test the debug page
• tagged @mapbox/map-design-team @mapbox/static-apis if this PR includes style spec API or visual changes
• tagged @mapbox/gl-native if this PR includes shader changes or needs a native port
• apply changelog label ('bug', 'feature', 'docs', etc) or use the label 'skip changelog'
• add an entry inside this element for inclusion in the mapbox-gl-js changelog: <changelog></changelog>

Briefly describe:
Fix sprite can't be written in relative path("sprite": "css/sprite") (#11921).

Changelog:

Bug fixes 🐞

Fix sprite can't be written in relative path("sprite": "css/sprite") (#11921).

#11921 skip changelog:

[CLAassistant]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution.

---

zjp369 seems not to be a GitHub user. You need a GitHub account to be able to sign the CLA. If you have already a GitHub account, please add the email address used for this commit to your account.
_{You have signed the CLA already but the status is still pending? Let us recheck it.}

[zjp369]

Bug fixes 🐞
Fix sprite can't be written in relative path("sprite": "css/sprite") (#11921).