Fix single item queries

Use 'VALUES' to bind ?urn to a specific value, instead of doing a
search-and-replace on the whole query. The latter approach was
generating invalid SPARQL, such as `GROUP BY <urn:...>`.
1 job for sam/tracker3 in 12 minutes and 7 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Test
failed #837708
flatpak
flatpak

00:12:07

 
Name Stage Failure
failed
flatpak Test
Running after_script
Uploading artifacts for failed job
Uploading artifacts...
org.gnome.Photos.Devel.flatpak: found 1 matching files

repo.tar: found 1 matching files

.flatpak-builder/build/gnome-photos/_flatpak_build/meson-logs/meson-log.txt: found 1 matching files

.flatpak-builder/build/gnome-photos/_flatpak_build/meson-logs/testlog.txt: found 1 matching files

Uploading artifacts to coordinator... ok
id=837708 responseStatus=201 Created token=7DsPPdqS
ERROR: Job failed: exit code 1