Nixpkgs on Darwin can currently build against either the macOS 10.12 or
the 11 SDK, but some vendored code in linuxkit expects at least the
macOS 12 SDK at build time, even though useful parts of that vendored
code still run on macOS 11, and the rest of linuxkit still runs on
10.12. This commit enables building against the macOS 11 SDK.
