Rollup merge of #39622 - alexcrichton:clean-dist, r=brson
Rollup merge of #39622 - alexcrichton:clean-dist, r=brson

rustbuild: Clean build/dist on `make clean`

Prevents stale artifacts from sticking around by accident!

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