Make lack of fonts.conf less of a footgun

We thought for a while to enable add a font allowed list also for Linux.

After #41776 (closed) we decided to finally do it, but it turned out it doesn't have the same outcome as having fonts.conf.

Main issues:

Arial, Courier New and Times New Roman are rendered (easy solution: add them to the allowed lists, they should be rendered with Arimo, Cousine and Tinos already; but we should find why this happens, and see if we should open an upstream Bug)
Stix Math becomes the default font again (fingerprintable by content with system-ui and system fonts!). This could be related to an upstream problem, since we hardcode Arimo, but it's ignored when it's bundled