deps: trim 'rand' dependencies

Looks like all we need is 'getrandom' and 'small_rng'. No need for
'std'.

Cargo.toml

@@ -27,4 +27,4 @@ name = "quickcheck"
 [dependencies]
 env_logger = { version = "0.8.2", default-features = false, optional = true }
 log = { version = "0.4", optional = true }
-rand = { version = "0.8", default-features = false, features = ["small_rng", "std"] }
+rand = { version = "0.8", default-features = false, features = ["getrandom", "small_rng"] }