fix(flake.nix): Revert using check node_modules

Author: jcs090218

flake.nix

@@ -33,8 +33,7 @@
                     echo "Welcome to the Emacs development shell with Eask!"
 
                     # Install if `node_modules` is missing
-                    if ! command -v eask >/dev/null 2>&1; then
-
+                    if [ ! -d node_modules ]; then
                       npm install --include=dev
                       export PATH="$PATH:$PWD/bin"
                       eask --version