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

[David Spickett via llvm-commits]

DavidSpickett wrote:

`issue-subscriber.yml` was the only one using `"$GITHUB_SHA"` instead of `main`,  I'm not sure why. Seems like `main` would be fine there too.

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