rustbuild: Pass `-Zsave-analysis` during tests
rustbuild: Pass `-Zsave-analysis` during tests

This is needed to ensure that the crates during a normal build are
shared with the crates during testing, otherwise they'll end up hasing
differently and we'll recompile crates like `core` during tests.

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