serve restic: fix immediate exit when not using stdio

In the lib/http refactor

    52443c2444 restic: refactor to use lib/http

We forgot to serve the data and wait for the server to finish. This is
not tested in the unit tests as it is part of the command line
handler.

Fixes #6644 Fixes #6647

cmd/serve/restic/restic.go

@@ -173,6 +173,7 @@ with a path of ` + "`/<username>/`" + `.
 				httpSrv.ServeConn(conn, opts)
 				return nil
 			}
+			s.Wait()
 			return nil
 		})
 	},