Auto merge of #38858 - ollie27:rustbuild_docs_std, r=alexcrichton
Auto merge of #38858 - ollie27:rustbuild_docs_std, r=alexcrichton

rustbuild: Stop building docs for std dependancies

Fixes: #38319

r? @alexcrichton

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