nixos: Write the directories creation script into store

When user writes a long list of directories, the original code
generates a very long activation script, which can cause an "Argument
list too long" error.

This try to fix the issue by writing the directories creation script
into nix store as an executable instead of repeating it multiple
times.

nixos.nix

@@ -87,11 +87,11 @@ in
 
     system.activationScripts =
       let
-        # Create a directory in persistent storage, so we can bind
+        # Script to create a directory in persistent storage, so we can bind
         # mount it. The directory structure's mode and ownership mirror those of
         # persistentStoragePath/dir;
         # TODO: Move this script to it's own file, add CI with shfmt/shellcheck.
-        mkDirWithPerms = persistentStoragePath: dir:
+        createDirectories = pkgs.writeShellScript "impermanence-create-directories"
           ''
             # Given a source directory, /source, and a target directory,
             # /target/foo/bar/bazz, we want to "clone" the target structure
@@ -108,10 +108,10 @@ in
             #   1. Ensure both /source/qualifiedPath and qualifiedPath exist
             #   2. Copy the ownership of the source path into the target path
             #   3. Copy the mode of the source path into the target path
-            (
-            # capture the nix vars into bash to avoid escape hell
-            sourceBase="${persistentStoragePath}"
-            target="${dir}"
+
+            # Get inputs from command line arguments
+            sourceBase="$1"
+            target="$2"
 
             # trim trailing slashes the root of all evil
             sourceBase="''${sourceBase%/}"
@@ -147,9 +147,12 @@ in
               # lastly we update the previousPath to continue down the tree
               previousPath="$currentTargetPath"
             done
-            )
           '';
 
+        mkDirWithPerms = persistentStoragePath: dir: ''
+          ${createDirectories} "${persistentStoragePath}" "${dir}"
+        '';
+
         # Build an activation script which creates all persistent
         # storage directories we want to bind mount.
         mkDirCreationScriptForPath = persistentStoragePath: