[rustbuild] --verbose should print the environment variables used by rustc wrapper

[nagisa]

running rustbuild with --verbose prints the commands executed, but one cannot copy and run them manually, because the wrapper depends on various environment variables.

Thus the environment variables should also be printed out so one could set them and be able to run the wrapper manually.

[est31]

👍 At least the relevant ones should be printed, like all the RUSTC_* ones. This would be very useful for debugging (did this locally already to debug something).

[nagisa]

@nikomatsakis hit it as well.

The most painless soln' would probably be something like --shell-on-fail flag which launches a shell with necessary variables set.