[flake] fix-bun with node_modules directory checking

flake.nix

@@ -75,8 +75,12 @@
               #!${pkgs.stdenv.shell}
               set -e
 
-              cd ./node_modules/better-sqlite3
-              ${pkgs.bun}/bin/bun install
+              DIR=./node_modules/better-sqlite3
+
+              if [ -d "$DIR" ]; then
+                cd $DIR
+                ${pkgs.bun}/bin/bun install
+              fi
               '';
             };
             compile = pkgs.writeShellApplication {