Document environment variables in README or TUTORIAL

[mqudsi]

This was my first time using cargo-bisect-rustc and I immediately jumped to --script because I was looking for a codegen regression and needed to inspect the generated asm. I'm ashamed to admit how much time I wasted trying to get it to work because I spent a long time barking up the wrong tree, thinking my invocation of cargo in bisect.sh was the problem (i.e. it wasn't resolving to the version being tested) when in fact the problem was that I wasn't aware each bisection run gets its own target directory.

I found that at minimum, CARGO_TARGET_DIR and CARGO_BUILD_TARGET need to be documented front-and-center (given that the output will be stored under $CARGO_TARGET_DIR/$CARGO_BUILD_TARGET/[release|debug]/....

Thanks for making this awesome tool in all cases, though!

[steffahn]

Thanks for writing this issue. I just wanted to bisect an ASM output issue, and thanks to your information here, it was really straightforward 🙂