An error when trying to sign a commit: no pinentry

**Issue**
Not possible to sign commits for now. When I try to make a commit with `gpgSign = true` in my config, I see the following error:

But it works if I first make a commit outside flatpak and then do another commit in the Github Desktop application.

**Possible cause**
If I try to launch this package in cli: `flatpak run --command=sh io.github.shiftey.Desktop`, and type `echo test | gpg --clear-sign`, I get this:
```
gpg: WARNING: server 'gpg-agent' is older than us (2.2.20 < 2.2.31)
gpg: Note: Outdated servers may lack important security fixes.
gpg: Note: Use the command "gpgconf --kill all" to restart them.
-----BEGIN PGP SIGNED MESSAGE-----
Hash: SHA256

test
gpg: signing failed: pinentry error
gpg: [stdin]: clear-sign failed: pinentry error
```

As I understand, to support gpg signing this flatpak package should include its own version of `pinentry`, at least for now.

You can find some discussion regarding this on other flatpak repos, e. g. https://github.com/flathub/org.gnome.Evolution/issues/12

## Additional info
Host OS: Fedora 35