message: feat(GUI): add env var to toggle fullscreen
  We add an environment variable to toggle Etcher in fullscreen.
  Closes:
  Change-Type: patch
  Changelog-Entry: Add environment variable to toggle fullscreen.
diff: @@ -40,10 +40,11 @@ electron.app.on('ready', () => {
  height: 380,
  useContentSize: true,
  show: false,
  - resizable: false,
  + resizable: Boolean(process.env.ETCHER_FULLSCREEN),
  maximizable: false,
  - fullscreen: false,
  - fullscreenable: false,
  + fullscreen: Boolean(process.env.ETCHER_FULLSCREEN),
  + fullscreenable: Boolean(process.env.ETCHER_FULLSCREEN),
  + kiosk: true,
  autoHideMenuBar: true,
  titleBarStyle: 'hidden-inset',
  icon: path.join(__dirname, '..', '..', 'assets', 'icon.png'),
  