sg: Use filesystem (not go-embed) contents for `sg migration ...` commands
Created by: efritz
Fixes #30205 (closed).
Test plan
- Installed sg from main
- Added migration
- Ran validate and see that it wasn't in the list (unintuitive)
- Cleared repo state
- Installed sg from branch
- Added migration
- Ran validate and it detected an out-of-date database (Intuitive)