Bump the cache version to 8
Bump the cache version to avoid unexpected behavior in older version of fontconfig.
There was no real incompatible features introduced in a cache. But the older version of fontconfig can't deal with variable fonts and then it failed. To avoid this breakage, you may want to choose one of options like:
Applying this change to fontconfig in host and remove all the fontconfig caches and then regenerate them by fc-cache linked with older one and new one. or simply run applications. You'll see two versions of caches in the cache directories.
Remove variable fonts on system and remove all the fontconfig caches too. then regenerate caches. You'll still see one version of caches there but without the entries of variable fonts.
After that, the problem should be gone.