gcr: use lib.meta.availableOn instead of isLinux

This commit uses `lib.meta.availableOn hostPlatform systemd`
(instead of `hostPlatform.isLinux`) to decide whether or not to
depend on systemd.

pkgs/development/libraries/gcr/default.nix

···
 , pango
 , libsecret
 , openssh
-, systemd
+, systemdSupport ? lib.meta.availableOn stdenv.hostPlatform systemd, systemd
 , gobject-introspection
 , wrapGAppsHook
 , gi-docgen
···
     pango
     libsecret
     openssh
-  ] ++ lib.optionals stdenv.isLinux [
+  ] ++ lib.optionals (systemdSupport) [
     systemd
   ];
 
···
     # We are still using ssh-agent from gnome-keyring.
     # https://github.com/NixOS/nixpkgs/issues/140824
     "-Dssh_agent=false"
-  ] ++ lib.optionals (!stdenv.isLinux) [
+  ] ++ lib.optionals (!systemdSupport) [
     "-Dsystemd=disabled"
   ];