fix: make installing Forgejo work again (#7452)

- The default engine is no longer guaranteed to be of the type `*xorm.Engine`, so instead return the interface `db.Engine`.
- Regression of forgejo/forgejo#7212

# Testing

1. Install a Forgejo instance via the setup screen.

Reviewed-on: https://codeberg.org/forgejo/forgejo/pulls/7452
Reviewed-by: Earl Warren <[e-mail]>
Co-authored-by: Gusted <[e-mail]>
Co-committed-by: Gusted <[e-mail]>

models/db/install/db.go

···
 import (
 	"forgejo.org/models/db"
 	"forgejo.org/modules/setting"
-
-	"xorm.io/xorm"
 )
 
-func getXORMEngine() *xorm.Engine {
-	return db.DefaultContext.(*db.Context).Engine().(*xorm.Engine)
+func getXORMEngine() db.Engine {
+	return db.DefaultContext.(*db.Context).Engine()
 }
 
 // CheckDatabaseConnection checks the database connection