Resolves freedomofpress#4100

you don't have an interactive prompt in the gui-updater, so you
shouldn't use a tty and --batch or --no-tty is meant for such tings.

--no-tty
     Make sure that the TTY (terminal) is never used for any output.
This option is needed in some cases because
     GnuPG sometimes prints warnings to the TTY if if --batch is used.

--batch
Use batch mode.  Never ask, do not allow interactive commands.

admin/securedrop_admin/__init__.py

@@ -642,7 +642,7 @@ def check_for_updates(args):
 
 
 def get_release_key_from_keyserver(args, keyserver=None, timeout=45):
-    gpg_recv = ['timeout', str(timeout), 'gpg', '--recv-key']
+    gpg_recv = ['timeout', str(timeout), 'gpg', '--batch', '--no-tty', '--recv-key']
     release_key = [RELEASE_KEY]
 
     # We construct the gpg --recv-key command based on optional keyserver arg.