Auto merge of #37151 - alexcrichton:fix-master, r=alexcrichton
Auto merge of #37151 - alexcrichton:fix-master, r=alexcrichton

rustbuild: Less panics in musl_root

Don't panic if the target wasn't configured.

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