For reasons I am not entirely clear on, when you use
`getrandom_v02::Error`, the `getrandom` crate pulls in `std` library.
But if you just directly implement what the `register_custom_getrandom!`
macro does, this goes away...

Fixes CI failure.

Full disclosure: I got the idea from
https://github.com/risc0/risc0/pull/3106/files#diff-fd010fc66ba8d918981d2705c0d10cd65d19db599969bb29bb14caffa62d3b65R95