problem fuzz+solver description to allow for env vars + validator

[Oli Lenehan]

Example to cover some scenarios.

[fuzz]
exec = ["deno", "run", "fuzz.ts"]
env = { ABC = 2 }

[judge]
exec = ["deno", "run", "validate.ts"]

Simple example to static compare

[judge]
exec = ["diff", "solve.txt", "-"]