Changes

ci(tiflash): fix sanitizer submodule auth (#4525)

## Background
The TiFlash repository recently introduced a new submodule,
`contrib/tici`. The sanitizer jobs fail during `git submodule update
--init --recursive` because the submodule is hosted in a private
repository and the current CI configuration does not provide
authentication for it.

## Summary
- allow private submodules such as contrib/tici to be fetched during
sanitizer job setup
 (commit: 366cc29)