Allow cargo_build_script to forward rustc args to build scripts

[UebelAndre]

rust-lang/cargo#9600 lead to the introduction of CARGO_ENCODED_RUSTFLAGS which was introduced to allow build scripts to access rust flags. This PR add this flag and sets --sysroot so crates like autocfg (which makes use of this flag here) is correctly able to make use of the generated rust_toolchain. This avoids issues like the following for users who create toolchains from separate rustc and rust_std artifacts.

Use --sandbox_debug to see verbose messages from the sandbox
error[E0107]: this struct takes 3 generic arguments but 2 generic arguments were supplied
  --> external/cargo_aliases__clap-3.0.0-beta.2/src/parse/matches/arg_matches.rs:77:22
   |
77 |     pub(crate) args: IndexMap<Id, MatchedArg>,
   |                      ^^^^^^^^ --  ---------- supplied 2 generic arguments
   |                      |
   |                      expected 3 generic arguments
   |
help: add missing generic argument
   |
77 |     pub(crate) args: IndexMap<Id, MatchedArg, S>,
   |                                             +++
 
error: aborting due to previous error

[illicitonion]

LGTM - could you add a test?

[UebelAndre]

LGTM - could you add a test?

Fixed a bug and added a test!

[illicitonion]

LGTM!