workspaceThumbnails: Replace loops with Array.find()

This is much more idiomatic and concise than iterating over
thumbnails until we find the one we are looking for.

GNOME/gnome-shell!710
6 jobs for !710 with some-cleanups in 4 minutes and 19 seconds (queued for 2 seconds)
detached
Status Job ID Name Coverage
  Review
passed #427717
check_commit_log

00:00:21

passed #427719
eslint

00:00:57

passed #427718
js_check

00:00:30

 
  Build
passed #427720
build

00:02:23

 
  Test
passed #427721
test

00:00:46

passed #427722
test-pot

00:00:57