Themes: Pass correct variable to `is_php_version_compatible()` in `wp_theme_update_row()`.

This applies to the Themes screen in Multisite network admin.

Follow-up to [48660].

Props pbiron, afragen.
Reviewed by peterwilsoncc, SergeyBiryukov.
Merges [48689] to the 5.5 branch.
Fixes #48507. See #48491.
Built from https://develop.svn.wordpress.org/branches/5.5@48690


git-svn-id: https://core.svn.wordpress.org/branches/5.5@48452 1a063a9b-81f0-0310-95a4-ce76da25c4cd