zsh: fix oh-my-zsh custom path expansion

Preserve shell-variable paths for programs.zsh.oh-my-zsh.custom while
still escaping values for a double-quoted shell assignment.

This restores runtime expansion for values like $HOME/extra/zsh, which
otherwise became a literal path after the previous escapeShellArg
change.

tests/modules/programs/zsh/default.nix

@@ -20,6 +20,7 @@
   zsh-history-path-zdotdir-variable = import ./history-path.nix "zdotdir-variable";
   zsh-history-substring-search = ./history-substring-search.nix;
   zsh-legacy-warning = ./legacy-warning.nix;
+  zsh-oh-my-zsh-custom = ./oh-my-zsh-custom.nix;
   zsh-plugins-completions-renamed = ./plugins-completions-renamed.nix;
   zsh-siteFunctions-mkcd = ./siteFunctions-mkcd.nix;
   zsh-plugins = ./plugins.nix;

tests/modules/programs/zsh/oh-my-zsh-custom.nix

@@ -0,0 +1,31 @@
+{
+  config = {
+    programs.zsh = {
+      enable = true;
+
+      oh-my-zsh = {
+        enable = true;
+        custom = "$HOME/extra/zsh";
+        theme = "sigma";
+      };
+    };
+
+    home.file.omz-zsh-theme = {
+      source = builtins.toFile "sigma.zsh-theme" ''
+        echo sigma
+      '';
+      target = "extra/zsh/themes/sigma.zsh-theme";
+    };
+
+    test.stubs = {
+      oh-my-zsh = { };
+      zsh = { };
+    };
+
+    nmt.script = ''
+      assertFileContains home-files/.zshrc 'ZSH_CUSTOM="$HOME/extra/zsh"'
+      assertFileContains home-files/.zshrc 'ZSH_THEME=sigma'
+      assertFileExists home-files/extra/zsh/themes/sigma.zsh-theme
+    '';
+  };
+}