Upgrade parking_lot dev-dependency to 0.12

We were previously stuck on 0.11 for MSRV reasons, but since
bumping nix's MSRV to Rust 1.56 this is no longer the case.

Author: rtzoeller

Cargo.toml

@@ -80,7 +80,7 @@ zerocopy = ["fs", "uio"]
 [dev-dependencies]
 assert-impl = "0.1"
 lazy_static = "1.4"
-parking_lot = "0.11.2"
+parking_lot = "0.12"
 rand = "0.8"
 tempfile = "3.3.0"
 semver = "1.0.7"