secret-backend: Check if portal is available

Before decising to use the file backend, check if the necessary portal
interface is available on the D-Bus.

Suggested by Patrick Griffis.
6 jobs for wip/dueno/local-file in 9 minutes and 57 seconds (queued for 1 second)
Status Job ID Name Coverage
  Build
passed #465009
fedora:asan

00:09:25

passed #465011
fedora:coverage

00:07:18

69.9%
passed #465010
fedora:ubsan

00:08:25

passed #465008
fedora:Werror

00:06:25

 
  Deploy
passed #465012
pages

00:00:31

passed pages:deploy

00:00:00