Fix file descriptor error for linux

Fixes: #1842 (closed) The function browse_to() in the obtain-secrets script should open up the Gitlab link in the browser, but it can't find the file descriptor on Linux as the Linux equivalent to OS X' open is xdg-open.

That's why I added a check whether xdg-open exists on the system and if yes, run xdg-open to open up the Gitlab tab in the system browser.