desktop: Failed signing (gpg2) using Flatpak

Describe the bug

error: cannot run gpg2: No such file or directory
error: gpg failed to sign the data
fatal: failed to write commit object

Version & OS

Version 2.9.0-linux4 (x64) flatpak

Fedora 29

Steps to reproduce the behavior

1. Setup git config to sign commits
2. Open a repo in GitHub Desktop
3. Make some changes
4. Commit

Expected behavior

A signed commit should be created

Actual behavior

Commit fails, as the flatpak version seems to be lacking permission to gpg2 (maybe just gpg on some systems).

Screenshots

Logs

Attach your logs by opening the Help menu and selecting Show Logs..., if applicable.

Additional context

Add any other context about the problem here.