Some images in iso-history.t.b.o become temporarily inaccessible

Reported by @anonym while RM'ing, experienced 403 while trying to access some Tails images:

• This was the first release after the switch to use git-annex adjusted branches.
• Seems that there were at least 2 instances of the issue:
  • One permanent, related to .git/annex/objects permissions, as well as the permissions of subdirectories and files.
  • One temporary, possibly related to the git rebase dance made by git-annex when updating an adjusted branch.
• The iso-history repo was updated right before the temporary instance, but waited for images to be listed on https://iso-hiustory.t.b.o with full size.

Cc: @foundations-team