flatpak: Fix the exiv2 build

The exiv2-0.26-trunk.tar.gz file doesn't exist at it's earlier
location. Instead, http://exiv2.org/download.html links to the 0.27.0
release.

Based on Darktable's Flathub manifest:
https://github.com/flathub/org.darktable.Darktable

!86
3 jobs for wip/rishi/fix-exiv2 in 15 minutes and 45 seconds (queued for 3 seconds)
Status Job ID Name Coverage
  Test
passed #178484
flatpak

00:14:38

 
  Review
passed #178485
review

00:00:33

passed #178486
allowed to fail manual
stop_review

00:00:32