build: make dependabot check daily for rust version upgrades

[coriolinus]

What's new in this PR

---

PR Submission Checklist for internal contributors

• The PR Title
  • conforms to the style of semantic commits messages¹ supported in Wire's Github Workflow²
  • contains a reference JIRA issue number like SQPIT-764
  • answers the question: If merged, this PR will: ... ³

1. https://sparkbox.com/foundry/semantic_commit_messages
2. https://github.com/wireapp/.github#usage
3. E.g. feat(conversation-list): Sort conversations by most emojis in the title #SQPIT-764.

[coriolinus]

I have no idea what's going wrong here, and it doesn't feel productive to keep investigating it. I'll leave this open until/unless someone wants to take it over, but I'm disclaiming it for now.