Fix engine test failure under flatpak

Geary.ImapDb.Database/utf8_case_insensitive_collation depends on a well
known UTF-8 collation order, but distros only ship C.UTF-8 by default
and Flatpak doesn't ship that, so support en_US.UTF-8 as a fallback.

See https://gitlab.com/freedesktop-sdk/freedesktop-sdk/issues/812
5 jobs for wip/composer-folks in 24 minutes and 59 seconds (queued for 1 second)
Status Job ID Name Coverage
  Build
passed #337436
fedora

00:11:03

passed #337437
ubuntu

00:11:39

 
  Test
passed #337438
flatpak

00:12:46

 
  Deploy
passed #337439
review

00:00:33

failed #337440
allowed to fail manual
stop_review

 
Name Stage Failure
failed
stop_review Deploy There has been a missing dependency failure
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