driver: Improve check for rustc arg
driver: Improve check for rustc arg

The rustc arg might not be exactly "rustc". It may be any path to a rustc
executable (especially if the RUSTC environment variable is set when
executing cargo). Rather check that it is a path with 'rustc' file stem.

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