mirage/mirage: PR #1586: call Printexc.record_backtrace early -- will be later overwritten by --backtrace (if provided)

@dfa6821

Stages:

• ❌ x86_64-debian-10-4.13 (10m53s)
• 🟡 arm64-debian-10-4.13

History:

• 14/10/2024 11:30 ✔️ @9f2919e (10m11s)
• 14/10/2024 09:26 ✔️ @6257fb7 (1h03m)
• 14/10/2024 08:49 ❌ @dfa6821 (10m53s)
• 14/10/2024 08:25 ❌ @77fcbbe (15m54s)