Remove 'doom sync' from shell.nix

Let user decide what they want to do upon starting the shell. We
accommodate the user that wants simple access to Emacs 26/27/HEAD.

core/core-packages.el

@@ -41,6 +41,13 @@ package's name as a symbol, and whose CDR is the plist supplied to its
 (defvar doom-disabled-packages ()
   "A list of packages that should be ignored by `use-package!' and `after!'.")
 
+(defvar doom-packages-file "packages"
+  "The basename of packages file for modules.
+
+Package files are read whenever Doom's package manager wants a manifest of all
+desired packages. They are rarely read in interactive sessions (unless the user
+uses a straight or package.el command directly).")
+
 
 ;;
 ;;; package.el

test/shell.nix

@@ -41,7 +41,5 @@ pkgs.stdenv.mkDerivation {
       cp -r "$EMACSDIR/.local/straight/repos" ./repos
     fi
     popd >/dev/null
-
-    doom sync
   '';
 }