Auto merge of #140732 - onur-ozkan:use-in-tree-rustfmt, r=Kobzol

make it possible to run in-tree rustfmt with `x run rustfmt`

Currently, there is no way to run in-tree `rustfmt` using `x fmt` or `x test tidy` commands. This PR implements `rustfmt` on `x run`, which allows bootstrap to run the in-tree `rustfmt`.

Fixes #140723

Author: bors