added symlink 'chromium-browser' for xdg-open to work as expected

```
$ xdg-open http://google.com
/run/current-system/sw/bin/xdg-open: line 455: firefox: command not found
/run/current-system/sw/bin/xdg-open: line 455: mozilla: command not found
/run/current-system/sw/bin/xdg-open: line 455: epiphany: command not found
/run/current-system/sw/bin/xdg-open: line 455: konqueror: command not found
/run/current-system/sw/bin/xdg-open: line 455: chromium-browser: command not found
/run/current-system/sw/bin/xdg-open: line 455: google-chrome: command not found
/run/current-system/sw/bin/xdg-open: line 455: links2: command not found
/run/current-system/sw/bin/xdg-open: line 455: links: command not found
/run/current-system/sw/bin/xdg-open: line 455: lynx: command not found
/run/current-system/sw/bin/xdg-open: line 455: w3m: command not found
xdg-open: no method available for opening 'http://google.com'
```