nixos/config: add defaultPackages option

readd perl (used in shell scripts), rsync (needed for NixOps) and strace (common debugging tool)

they where previously removed in https://github.com/NixOS/nixpkgs/pull/91213

Co-authored-by: Timo Kaufmann <timokau@zoho.com>
Co-authored-by: 8573 <8573@users.noreply.github.com>

nixos/doc/manual/release-notes/rl-2009.xml

@@ -1016,7 +1016,11 @@ services.transmission.settings.rpc-bind-address = "0.0.0.0";
     <para>
      Nginx module <literal>nginxModules.fastcgi-cache-purge</literal> renamed to official name <literal>nginxModules.cache-purge</literal>.
      Nginx module <literal>nginxModules.ngx_aws_auth</literal> renamed to official name <literal>nginxModules.aws-auth</literal>.
-      The packages <package>perl</package>, <package>rsync</package> and <package>strace</package> were removed from <option>systemPackages</option>. If you need them, install them again with <code><xref linkend="opt-environment.systemPackages"/> = with pkgs; [ perl rsync strace ];</code> in your <filename>configuration.nix</filename>.
+    </para>
+   </listitem>
+   <listitem>
+    <para>
+      The option <option>defaultPackages</option> was added. It installs the packages <package>perl</package>, <package>rsync</package> and <package>strace</package> for now. They were added unconditionally to <option>systemPackages</option> before, but are not strictly necessary for a minimal NixOS install. You can set it to an empty list to have a more minimal system. Be aware that some functionality might still have an impure dependency on those packages, so things might break.
     </para>
    </listitem>
    <listitem>

nixos/modules/config/system-path.nix

@@ -41,6 +41,12 @@ let
       pkgs.zstd
     ];
 
+    defaultPackages = map (pkg: setPrio ((pkg.meta.priority or 5) + 3) pkg)
+      [ pkgs.perl
+        pkgs.rsync
+        pkgs.strace
+      ];
+
 in
 
 {
@@ -63,6 +69,21 @@ in
         '';
       };
 
+      defaultPackages = mkOption {
+        type = types.listOf types.package;
+        default = defaultPackages;
+        example = literalExample "[]";
+        description = ''
+          Set of packages users expect from a minimal linux istall.
+          Like systemPackages, they appear in
+          /run/current-system/sw.  These packages are
+          automatically available to all users, and are
+          automatically updated every time you rebuild the system
+          configuration.
+          If you want a more minimal system, set it to an empty list.
+        '';
+      };
+
       pathsToLink = mkOption {
         type = types.listOf types.str;
         # Note: We need `/lib' to be among `pathsToLink' for NSS modules
@@ -102,7 +123,7 @@ in
 
   config = {
 
-    environment.systemPackages = requiredPackages;
+    environment.systemPackages = requiredPackages ++ config.environment.defaultPackages;
 
     environment.pathsToLink =
       [ "/bin"