Fix upgrade script

[MayeulC]

The destination directory for upgrade source was wrong, so upgrading would reinstall the previous version.

Problem

Upgrading didn't actually upgrade mautrix_signal source code

Solution

Debugging led me to this issue

PR Status

• [X] Code finished.
• [ ] Tested with Package_check.
• [X] Fix or enhancement tested.
• [X] Upgrade from last version tested.
• [X] Can be reviewed and tested.

Package_check results

---

If you have access to App Continuous Integration for packagers you can provide a link to the package_check results like below, replacing '-NUM-' in this link by the PR number and USERNAME by your username on the ci-apps-dev. Or you provide a screenshot or a pastebin of the results