Upgrade/Install: When creating the `.htaccess` file in...

Upgrade/Install: When creating the `.htaccess` file in `insert_with_markers()`, make sure it has `0644` as a minimum set of permissions.

Props rcutmore, dd32, i3anaan.
Fixes #40572.
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git-svn-id: https://core.svn.wordpress.org/trunk@46805 1a063a9b-81f0-0310-95a4-ce76da25c4cd