fix(core): search json path is incorrect when config.outDir is absolute

[liuwenzhuang]

Summary

Serve search json will be failed when config.outDir is a absolute path, use same path join strategy from writeSearchIndex function instead.

Related Issue

Checklist

• Tests updated (or not required).
• Documentation updated (or not required).

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[SoonIter]

Thanks❤, this indeed appears to be a corner case
Feel free to further test with the config.outputDir: absolutePath setting, submit PRs, and add more test cases to ensure stability of the functionality.