Ensure pinentry-gtk2 run by Seahorse has the correct $DISPLAY set (refs: #12733).
As discussed in more details on https://bugs.debian.org/869416, there's a bug somewhere in the Seahorse → gnupg-agent → pinentry-gtk2 communication, that leads to pinentry-gtk2 being invoked with the $DISPLAY environment variable being unset. Strangely, this does not happen for gpg → gnupg-agent → pinentry-gtk2. Now that upstream is aware of the problem, let's apply a temporary workaround to Tails until the root cause is fixed.
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