Not possible to disable git ls-tree for submodules

It's not possible to disable git ls-tree through configurations for submodules. Setting checkout false makes Buildstream still connect to possible flaky remote servers leading to CI failures. We noticed this through https://gitlab.com/freedesktop-sdk/freedesktop-sdk/-/jobs/130206659

To upload designs, you'll need to enable LFS and have an admin enable hashed storage. More information