Rollup merge of #39478 - alexcrichton:add-xpy, r=japaric
Rollup merge of #39478 - alexcrichton:add-xpy, r=japaric

rustbuild: Add x.py to source tarballs

We should be sure to add our build system entry point!

Closes #39476

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