No pinentry prompt for PGP signing

Perhaps this is connected to #12.

The problem is, that I do not get a password prompt when trying to PGP sign or encrypt with a passphrase protected key. In the GUI it says: "Secret key not found".

My observation is, that if there is a gpg-agent running, it never works and if not it is kind of random, it worked once in ten tries. Didn't manage to detect a course.

[Here](https://github.com/flathub/io.github.shiftey.Desktop/issues/22#issuecomment-1586333404) I found a workaround by setting `socket=gpg-agent` permission to `false`, but I think this not a real solution.

I tried to play around with
```
LANG=C CAMEL_DEBUG=gpg flatpak run org.gnome.Evolution
```
but cannot see anything enlightening in the output.

Any advice how to get closer to the problem here?

Version used for latest tests:
```sh
$> flatpak info org.gnome.Evolution

Evolution - Ihre E-Mails, Kontakte und Terminplanung verwalten

Kennung: org.gnome.Evolution
Ref: app/org.gnome.Evolution/x86_64/stable
Architektur: x86_64
Zweig: stable
Version: 3.48.4
Lizenz: GPL-2.0+ and GFDL-1.3
Ursprung: flathub
Sammlung: org.flathub.Stable
Installation: system
Installiert: 61,8 MB
Laufzeitumgebung: org.gnome.Platform/x86_64/44
Sdk: org.gnome.Sdk/x86_64/44

Commit: 45cacdba238ad33c9e69f85a35fe73e631a2ce797322050a9ed545685aab4c59
Parent: 18b94f8428db3edbb8dc1f0e45e83445867ff35c0b13de01f30ca4bb7520e99a
Betreff: Update to 3.48.4 (ee1be398)
Datum: 2023-07-24 13:11:49 +0000
```