search-engine: Query file system to determine remoteness

Querying the file cache in is_recursive_search() does not guarantee that
the file system field will be populated with a value, and somehow
results in data races, in turn resulting in random crashes/file list
corruption.

Hopefully fixes #640,
closes #632 and resolves
#602
1 job for master in 2 minutes and 50 seconds (queued for 3 seconds)
Status Job ID Name Coverage
  Test
passed #111341
flatpak

00:02:50