The `wp-content/upgrade` directory does not exist initially after installation, so the Site Health check could not determine the available disk space until the directory was subsequently created during an update.
By testing `WP_CONTENT_DIR` instead, the check can complete successfully.
This also brings consistency with similar checks in `_unzip_file_ziparchive()` and `_unzip_file_pclzip()`.
Follow-up to [55720], [56401].
Props wbdv, khokansardar, mi5t4n, SergeyBiryukov.
Fixes#61602.
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