Thumbnail: Avoid updating an outdated thumbnail

If the thumbnail is being relocated after the thumbnailer generated an
actual thumbnail, ensure that we won't override the thumbnailer's result
5 jobs for fix_thumbnailer_race in 21 minutes and 3 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Test
passed #290694
amd64 debian
asan-ubsan

00:10:10

passed #290691
amd64 debian
linux

00:10:04

passed #290692
win32
win32

00:12:18

passed #290695
win64
win64

00:08:44

failed #290693
win64
win64

00:06:29