rustbuild: Compile the error-index in stage 2
rustbuild: Compile the error-index in stage 2

Right now we comiple rustdoc in stage 2 and the error index in stage 0, which
ends up compiling rustdoc twice! To avoid compiling rustdoc twice (which takes
awhile) let's just compile it once in stage 2.

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