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

[zjp369]

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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:

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[zjp369]

Bug fixes 🐞 Fix sprite can't be written in relative path("sprite": "css/sprite") (https://github.com/mapbox/mapbox-gl-js/issues/11921).