networkAgent: Add missing Uint8Array => string conversion

When promisifying async operations in commit 764527c8c, the
finish function for read_line_async() was sneakily changed from
read_line_finish_utf8() to read_line_finish().

That is, the call returns a Uint8Array now that requires an
explicit conversion to string.

https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/1407

js/ui/components/networkAgent.js

@@ -494,7 +494,7 @@ var VPNRequestHandler = class {
             return;
         }
 
-        this._vpnChildProcessLineOldStyle(line);
+        this._vpnChildProcessLineOldStyle(ByteArray.toString(line));
 
         // try to read more!
         this._readStdoutOldStyle();