Add caching to the CI fuzzing target

[Shnatsel]

CI fuzz job takes a long time not because it fuzzes for a while, but because it takes a long time to compile.

It should be made to use caching, just like the rest of CI.

[fintelia]

Fixed in #2157. The build itself isn't cached, but the installation of cargo-fuzz is. Also switched to "check" instead of a full build which further sped things up