flake.nix: run tests against pinned nixpkgs

and migrate to the new runTest, which evaluates much faster.

tests/external.nix

@@ -14,18 +14,19 @@
 #  You should have received a copy of the GNU General Public License
 #  along with this program. If not, see <http://www.gnu.org/licenses/>
 
-{ pkgs ? import <nixpkgs> {}, ...}:
-
-pkgs.nixosTest {
+{
   name = "external";
+
   nodes = {
-    server = { config, pkgs, ... }:
+    server = { pkgs, ... }:
         {
             imports = [
                 ../default.nix
                 ./lib/config.nix
             ];
 
+            environment.systemPackages = with pkgs; [ netcat ];
+
             virtualisation.memorySize = 1024;
 
             services.rsyslogd = {
@@ -86,7 +87,7 @@ pkgs.nixosTest {
               };
             };
         };
-      client = { nodes, config, pkgs, ... }: let
+      client = { nodes, pkgs, ... }: let
         serverIP = nodes.server.networking.primaryIPAddress;
         clientIP = nodes.client.networking.primaryIPAddress;
         grep-ip = pkgs.writeScriptBin "grep-ip" ''
@@ -341,8 +342,7 @@ pkgs.nixosTest {
       };
     };
 
-  testScript = { nodes, ... }:
-      ''
+  testScript = ''
       start_all()
 
       server.wait_for_unit("multi-user.target")
@@ -350,7 +350,7 @@ pkgs.nixosTest {
 
       # TODO put this blocking into the systemd units?
       server.wait_until_succeeds(
-          "set +e; timeout 1 ${nodes.server.nixpkgs.pkgs.netcat}/bin/nc -U /run/rspamd/rspamd-milter.sock < /dev/null; [ $? -eq 124 ]"
+          "set +e; timeout 1 nc -U /run/rspamd/rspamd-milter.sock < /dev/null; [ $? -eq 124 ]"
       )
 
       client.execute("cp -p /etc/root/.* ~/")