Fix gulp-notify errors in CI build output

[chrahunt]

Running gulp build and gulp build-release results in this output:

[17:07:07] gulp-notify: [Gulp notification] Built /home/runner/work/TagProReplays/TagProReplays/build/js/background.js
[17:07:07] gulp-notify: [Error in notifier] Error in plugin "gulp-notify"
Message:
    notify-send must be installed on the system.

One for each compiled JS file. We can probably suppress these errors, or not use gulp-notify for non-watch Gulp tasks, since they are really only relevant there.