cachix-agent: fix crash calling security

One of cachix-agent's dependencies, `hs-certificate`, makes calls to
`security`. This lives in `/usr/bin`, which isn't available from
launchd. This commit makes the system paths available to cachix-agent.

Fixes LnL7#924.

modules/services/cachix-agent.nix

@@ -58,7 +58,7 @@ in {
         exec ${cfg.package}/bin/cachix deploy agent ${cfg.name}
       '';
 
-      path = [ config.nix.package pkgs.coreutils ];
+      path = [ config.nix.package config.environment.systemPath ];
 
       environment = {
         NIX_SSL_CERT_FILE = "${pkgs.cacert}/etc/ssl/certs/ca-bundle.crt";