ci: Enable radv-fossils

Disabled in !14364 (merged). This was addressed in the mesa-valve-ci project, but has not been brought upstream yet.

The root cause of this error was a GitHub brownout of unauthenticated git:// access.

The commit that should fix the 504 errors is,

https://github.com/ValveSoftware/Fossilize/commit/16fba1b8b5d9310126bb02323d7bae3227338461

I took the liberty to build the latest master of Fossilize, since we were a bit out of date.