flatpak: Fix the exiv2 build

The exiv2-0.27.0-Source.tar.gz file doesn't exist at it's earlier
location. Instead, http://exiv2.org/download.html links to the 0.27.1
release.

!103
3 jobs for wip/rishi/fix-exiv2-build in 15 minutes and 28 seconds (queued for 1 second)
Status Job ID Name Coverage
  Test
passed #313225
flatpak

00:14:18

 
  Review
passed #313226
review

00:00:33

passed #313227
allowed to fail manual
stop_review

00:00:36