[llvm] [GitHub][workflows] Replace curl with sparse checkout (PR #78303)

[Aiden Grossman via llvm-commits]

https://github.com/boomanaiden154 approved this pull request.

LGTM. I think this was something I suggested on at least one of the reviews. This will definitely help keep things a bit more consistent and prevent us from running into footguns like curl not failing without `--fail`. Thanks!

https://github.com/llvm/llvm-project/pull/78303