ci: build with nix on macos (and push to cachix) (#2610)

The idea is to ensure the macos nix build works, and to
build a cache in cachix to help local development on macos.

default.nix

@@ -94,10 +94,6 @@ rec {
   postgrestPackage =
     lib.dontCheck postgrest;
 
-  # Static executable.
-  postgrestStatic =
-    lib.justStaticExecutables (lib.dontCheck (staticHaskellPackage name src));
-
   # Profiled dynamic executable.
   postgrestProfiled =
     lib.enableExecutableProfiling (
@@ -123,10 +119,6 @@ rec {
   devTools =
     pkgs.callPackage nix/tools/devTools.nix { inherit tests style devCabalOptions hsie withTools; };
 
-  # Docker images and loading script.
-  docker =
-    pkgs.callPackage nix/tools/docker { postgrest = postgrestStatic; };
-
   # Load testing tools.
   loadtest =
     pkgs.callPackage nix/tools/loadtest.nix { inherit withTools; };
@@ -158,4 +150,12 @@ rec {
 
   withTools =
     pkgs.callPackage nix/tools/withTools.nix { inherit devCabalOptions postgresqlVersions postgrest; };
+} // pkgs.lib.optionalAttrs pkgs.stdenv.isLinux rec {
+  # Static executable.
+  postgrestStatic =
+    lib.justStaticExecutables (lib.dontCheck (staticHaskellPackage name src));
+
+  # Docker images and loading script.
+  docker =
+    pkgs.callPackage nix/tools/docker { postgrest = postgrestStatic; };
 }