Unbreak unstable (std/random missing after dup validator protection)

[mratsim]

Btw is there a reason to use Nim's std/random instead of the Nimbus-wide (cryptographic) RNG?

[tersec]

I removed the stdlib random import in #2240 and it seems to have worked there, but for some reason it broke later. There's not a compelling reason to use the stdlib random in nimbus_beacon_node, no, but it's more important to unbreak unstable.

[zah]

Hmm, I wonder how the previous PR passed CI. I've pushed a follow-up commit that removes std/random again here:
960666d