Add RUSTC as env_var to buildscripts

[acmcarther]

Fixes #32

Probably will be broken in CI. Will fix.

[acmcarther]

Thanks @SirVer for the exact fix suggestion, looks correct (and is probably correct +- oddities in examples)!

[acmcarther]

Oh, it just works.

I guess this expresses the "chicken-or-egg" problem of new features in the current project structure. We know that this didn't break the existing examples, but what we really want to know is if this works with a crate where it's actually used.

... but I can't add that example without this feature.

If I can think of a reliable way to defer the actual execution of the example generation until after the code is submitted (so it doesn' t clutter up diffs) the example repo can be merged back in. That would let us atomically add examples and features.

[acmcarther]

This is broken in a couple of ways (not sure how CI missed it):

Exhibit A:

 bazel build //third_party/cargo:all                                                                     [acm-rm-shame]
ERROR: /usr/local/google/home/acmcarther/.cache/bazel/_bazel_acmcarther/790a941678244b6c942ed3c699de3d11/external/raze__bindgen__0_35_0/BUILD.bazel:45:11: in cmd attribute of genrule rule @raze__bindgen__0_35_0//:bindgen_build_script_executor: label '@rust_linux_x86_64//:rustc' in $(location) expression is not a declared prerequisite of this rule
ERROR: Analysis of target '//third_party/cargo:cargo_bin_bindgen' failed; build aborted: Analysis of target '@raze__bindgen__0_35_0//:bindgen_build_script_executor' failed; build aborted
INFO: Elapsed time: 16.346s

(rustc needs to be added to tools)

In addition, we can't just use @rust_linux_x86_64//:rustc, as that compiler is target specific.

Going to roll back for now.