treewide: libressl.nc -> netcat

Quote from NixOS 25.11 release notes:

> `meta.mainProgram` is now used to determine the `NIX_MAIN_PROGRAM` environment variable. This means that changing it can now lead to a package rebuild.

And `netcat` is:

```
 netcat = libressl.nc.overrideAttrs (old: { 
   meta = old.meta // { 
     description = "Utility which reads and writes data across network connections — LibreSSL implementation"; 
     mainProgram = "nc"; 
   }; 
 });
```

`netcat` and `libressl.nc` are 2 different derivations now, use just one
 of them in nixpkgs for consistency.

This also fixes lots of warnings if `virtualisation.libvirtd.enable = true`:

```
pkgs.buildEnv warning: colliding subpath (ignored): `/nix/store/1mcayh9rmxmjcbmm6swkkyr59rjl66vc-libressl-4.2.1-nc/bin/nc' and `/nix/store/x1vmfpisbd494yykmvjkhvh3dplsnjhg-libressl-4.2.1-nc/bin/nc'
...
```

nixos/modules/virtualisation/libvirtd.nix

@@ -421,7 +421,7 @@ in
       # this file is expected in /etc/qemu and not sysconfdir (/var/lib)
       etc."qemu/bridge.conf".text = lib.concatMapStringsSep "\n" (e: "allow ${e}") cfg.allowedBridges;
       systemPackages = with pkgs; [
-        libressl.nc
+        netcat
         config.networking.firewall.package
         cfg.package
         cfg.qemu.package

nixos/tests/ceph-multi-node.nix

@@ -53,7 +53,7 @@ let
         sudo
         ceph
         xfsprogs
-        libressl.nc
+        netcat
       ];
 
       boot.kernelModules = [ "xfs" ];

nixos/tests/fail2ban.nix

@@ -14,7 +14,7 @@
   nodes.client = _: {
     environment.systemPackages = [
       pkgs.sshpass
-      pkgs.libressl.nc
+      pkgs.netcat
     ];
 
   };

nixos/tests/jool.nix

@@ -24,7 +24,7 @@ let
           printf 'HTTP/1.0 200 OK\n'
           printf 'Content-Length: ${toString (1 + builtins.stringLength msg)}\n'
           printf '\n${msg}\n\n'
-        } | ${pkgs.libressl.nc}/bin/nc -${toString ip}nvl 80
+        } | ${pkgs.netcat}/bin/nc -${toString ip}nvl 80
         done
       '';
     };

nixos/tests/samba-wsdd.nix

@@ -40,7 +40,7 @@
     server_wsdd.wait_for_unit("samba-wsdd")
 
     client_wsdd.wait_until_succeeds(
-        "echo list | ${pkgs.libressl.nc}/bin/nc -N -U /run/wsdd/wsdd.sock | grep -i SERVER-WSDD"
+        "echo list | ${pkgs.netcat}/bin/nc -N -U /run/wsdd/wsdd.sock | grep -i SERVER-WSDD"
     )
   '';
 }

pkgs/by-name/hy/hydra/package.nix

@@ -38,7 +38,7 @@
   mdbook,
   foreman,
   python3,
-  libressl,
+  netcat,
   cacert,
   glibcLocales,
   meson,
@@ -208,7 +208,7 @@ stdenv.mkDerivation (finalAttrs: {
     foreman
     glibcLocales
     python3
-    libressl.nc
+    netcat
     nix-eval-jobs
     openldap
     postgresql