build(nix): macos-nextest runs the tests (doCheck = true)

The macos-nextest check was built with doCheck = false like the Windows one, so crane printed
'doCheck is not set, nextest tests will not run' and the check ran no test. It now uses
doCheck = true, as linux-nextest does, so the macOS unit tests run in the nix sandbox of a
darwin machine (the owner asked: can nextest run on a Mac with nix-darwin; if so, keep it).

Verified on the Mac Mini (aarch64-darwin, nix, no komorebi daemon started):
nix build .#checks.aarch64-darwin.macos-nextest exits 0 in about 1 minute with the cached
dependencies; nextest reports 'Summary 383 tests run: 383 passed, 1 skipped'. This is the
komorebi-macos library's tests; the cargo test run of the other packages (753 tests in all) was
done outside nix earlier. Not run: the other macOS checks again after this change (it only
touches this one check's arguments).

nix/macos/flake-module.nix

@@ -56,7 +56,7 @@ in
           }
         );
 
-        macos-nextest = build.craneLib.cargoNextest build.individualCrateArgs;
+        macos-nextest = build.craneLib.cargoNextest (build.individualCrateArgs // { doCheck = true; });
 
         macos-nix-options-validation = import ./tests { inherit pkgs; };