build: use macOS 11 SDK (Xcode 12.2)

This should be sufficient to support building for Apple ARM when
cross-compiling.

contrib/guix/README.md

@@ -249,7 +249,7 @@ details.
   Set the path where _extracted_ SDKs can be found. This is passed through to
   the depends tree. Note that this is should be set to the _parent_ directory of
   the actual SDK (e.g. `SDK_PATH=$HOME/Downloads/macOS-SDKs` instead of
-  `$HOME/Downloads/macOS-SDKs/Xcode-12.1-12A7403-extracted-SDK-with-libcxx-headers`).
+  `$HOME/Downloads/macOS-SDKs/Xcode-12.2-12B45b-extracted-SDK-with-libcxx-headers`).
 
   The path that this environment variable points to **must be a directory**, and
   **NOT a symlink to a directory**.