bump test-cargo-miri dependencies

Author: RalfJung

test-cargo-miri/Cargo.lock

@@ -15,7 +15,7 @@ issue_1691 = { path = "issue-1691" }
 issue_1705 = { path = "issue-1705" }
 
 [dev-dependencies]
-rand = { version = "0.7", features = ["small_rng"] }
+rand = { version = "0.8", features = ["small_rng"] }
 getrandom = { version = "0.2" }
 serde_derive = "1.0" # not actually used, but exercises some unique code path (`--extern` .so file)