lib: Ignore USB sticks if they are read-only

For the purposes of copying an app to a USB stick, we only want to try
if the USB stick is not mounted read-only. This can often be the case
when (for example) running from a live USB.

Signed-off-by: Philip Withnall <withnall@endlessm.com>
1 job for usb-app-copying in 17 minutes and 16 seconds (queued for 2 seconds)
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build-gnome-software

00:17:16