Auto merge of #38809 - alexcrichton:rustbuild-fresh, r=brson
Auto merge of #38809 - alexcrichton:rustbuild-fresh, r=brson

rustbuild: Fix a few rebuilding issues

Did a bit of investigation and found a few small unrelated issues, but this should help clean up a lot of errors we've been seeing locally.

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