fix(nix/pkgs/spin-plugin): Fix manifest os field mapping

The [implementation][0] uses `macos`, not `osx`. Also SIP006 uses
`macos` in one of the example URLs. Additionally, the authoritative
`spin-plugin-manifest-schema-0.1.json` was [updated][1] as well.

PR to fix the typo upstream: spinframework/spin#3011

[0]: https://github.com/fermyon/spin/blob/v3.1.2/crates/plugins/src/manifest.rs#L113
[1]: spinframework/spin-plugins#10

Author: PetarKirov

nix/pkgs/spin-plugin/default.nix

@@ -18,7 +18,7 @@ let
   os =
     {
       linux = "linux";
-      darwin = "osx";
+      darwin = "macos";
     }
     .${hostPlatform.parsed.kernel.name};