rustbuild: Fix submodules for out of tree builds
rustbuild: Fix submodules for out of tree builds

Be sure to put the git command into the right directory when we run the
submodule management information.

Closes #31619

graydon/rust,GBGamer/rust,aidancully/rust,aidancully/rust,aidancully/rust,GBGamer/rust,aidancully/rust,GBGamer/rust,graydon/rust,GBGamer/rust,graydon/rust,GBGamer/rust,aidancully/rust,graydon/rust,graydon/rust,graydon/rust,GBGamer/rust,aidancully/rust,GBGamer/rust,GBGamer/rust