mirage/mirage: PR #1488: move runtime keys back (as strings) to mirage_runtime_key

@ec9a23a

Stages:

• ❌ x86_64-debian-10-4.13 (5m48s)
• ❌ arm64-debian-10-4.13 (5m04s)

History:

• 12/11/2023 23:41 ✔️ @17a880b (7m43s)
• 12/11/2023 23:08 ✔️ @4c31a09 (5m38s)
• 12/11/2023 23:02 ❌ @ec9a23a (5m48s)