Dogfood `CARGO_REGISTRIES_CRATES_IO_PROTOCOL=sparse` in CI

[jyn514]

Sparse-registry was recently stabilized and cargo itself has started dogfooding it in CI: rust-lang/cargo#11632. We should do the same in rust-lang/rust, both to speed up CI and to test out the feature.

I think around

rust/src/ci/run.sh

Lines 129 to 130 in beefcf8

	export COMPILETEST_NEEDS_ALL_LLVM_COMPONENTS=1

is the right place to set this; we should verify by inspection that the feature is being used in CI before merging.