Pinned nixpkgs for local shell

Ensure a local shell is reproducible and doesn't need internet access to
be activated.

To make room for the new option, 'local_shell' is renamed
'shell_nix.enabled' and definition is moved. The new option
'activation_command' is needed to implement shells in a separate module.

Author: Julow

modules/shell_nix.nix

@@ -0,0 +1,38 @@
+{ config, pkgs, lib, ... }:
+
+with lib;
+
+let
+  conf = config.shell_nix;
+
+  nixpkgs_incl = if conf.nixpkgs != null then
+    "-I nixpkgs=${escapeShellArg conf.nixpkgs}"
+  else
+    "";
+
+in {
+  options.shell_nix = {
+    enabled = mkOption {
+      type = types.bool;
+      default = false;
+      description = "Whether use 'shell.nix' in the workspace's tree.";
+    };
+
+    nixpkgs = mkOption {
+      type = types.nullOr types.path;
+      default = null;
+      description = "Pinned nixpkgs for evaluating the local shell.";
+    };
+  };
+
+  config = mkIf conf.enabled {
+    activation_command = ''
+      if [[ -e ./shell.nix ]]
+      then exec nix-shell ./shell.nix ${nixpkgs_incl} --run ${
+        escapeShellArg config.command
+      }
+      else ${config.command}
+      fi
+    '';
+  };
+}

workspaces.nix

@@ -43,12 +43,12 @@ let
           "Directory for per-workspace cache, relative to the home directory. Used to store history files and other unimportant things.";
       };
 
-      local_shell = mkOption {
-        type = types.bool;
-        default = false;
-        description = "Whether to build and use a local 'shell.nix'.";
+      activation_command = mkOption {
+        type = types.str;
+        default = config.command;
+        description =
+          "Command run at the end of the activation script. The default is to run 'command'.";
       };
-
     };
 
     config = {
@@ -70,23 +70,15 @@ let
           base_module
           default_name # Base modules
           modules/git.nix
-          modules/vim.nix
+          modules/shell_nix.nix
           modules/tools.nix
+          modules/vim.nix
           modules/xdg.nix
           configuration # User configuration
         ];
       };
     in modules.config;
 
-  make_activate_command = w:
-    if w.local_shell then ''
-        if [[ -e ./shell.nix ]]
-        then exec nix-shell ./shell.nix --run ${escapeShellArg w.command}
-        else ${w.command}
-        fi
-    '' else
-      w.command;
-
   stdenv = pkgs.stdenvNoCC;
 
   # Generate the 'workspace-init' and 'workspace-activate' script for the
@@ -105,7 +97,7 @@ let
       activation_script = ''
         mkdir -p "$HOME/${w.cache_dir}"
         ${w.activation_script}
-        ${make_activate_command w}
+        ${w.activation_command}
       '';
 
       # Similar to 'pkgs.writeShellScriptBin', inlined to avoid generating many