rustbuild: Don't package libstd twice
rustbuild: Don't package libstd twice

Looks like the packaging step for the standard library was happening
twice on CI, but it only needs to happen once! The `Analysis` packaging
step accidentally packaged `Std` instead of relying on compiling `Std`,
which meant that we ended up packaging it twice erroneously.

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