warn if absolute path to rustc is used

[matthiaskrgr]

I recently ran into an interesting™ situation where I passed an absolute rustc path into CBR, like --script=/home/matthias.../.rustup/.../rustc
This prevents cbr from being able to dynamically alter the RUSTC version used (I guess) since rustc is no longer gotten through the the rustup wrapper or whatever, meaning that cbd will potentially download an infinite number of nightlies because it never finds a change in rustc $file behaviour because the rustc used is always the same 😆