Wrap emulator exec script in double quotes

[aalej]

Wrap the script argument of firebase emulators:exec command.

Related issue firebase/firebase-tools#7476

On a Windows machine, when running the command below, the emulators do not start

firebase --project demo-friendly-eats emulators:exec --import ./app-data-seed --ui 'ng serve'

Might be related to how Windows treats single quotes https://stackoverflow.com/a/51090074.

Running the command below, emulators were able to start:

firebase --project demo-friendly-eats emulators:exec --import ./app-data-seed --ui "ng serve"