fix: add a missing user field in cron file (aka fix the cron again o/)

[Tagadda]

Problem

/etc/cron.d/mastodon is a "system cron file" and the syntax slightly differ from other cron files. It has a user field which is missing, and the cron tasks fail. See on the forum

Solution

Fix the cron file again :partying_face:

PR Status

• [x] Code finished and ready to be reviewed/tested
• [ ] The fix/enhancement were manually tested (if applicable)

Automatic tests

Automatic tests can be triggered on https://ci-apps-dev.yunohost.org/ after creating the PR, by commenting "!testme", "!gogogadgetoci" or "By the power of systemd, I invoke The Great App CI to test this Pull Request!". (N.B. : for this to work you need to be a member of the Yunohost-Apps organization)

[Tagadda]

!testme

[yunohost-bot]

:v: