app: Update a FIXME comment to mention the upstream bug

And hence provide at least a little bit of hope that this doesn’t become
permanent technical debt.

Signed-off-by: Philip Withnall <withnall@endlessm.com>

Closes: #2296
Approved by: matthiasclasen
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Gitea Version: 1.28.0+dev-477-g8b6ad49a5f