migrations: add temporary hack for s3_layout

The migration will be removed after the next restic release anyways.
Thus, there's no need for a clean implementation.

internal/repository/s3_backend.go

@@ -0,0 +1,12 @@
+package repository
+
+import (
+	"github.com/restic/restic/internal/backend"
+	"github.com/restic/restic/internal/backend/s3"
+)
+
+// AsS3Backend extracts the S3 backend from a repository
+// TODO remove me once restic 0.17 was released
+func AsS3Backend(repo *Repository) *s3.Backend {
+	return backend.AsBackend[*s3.Backend](repo.be)
+}