Bug 1624706 - Pass through RUSTC in RunCbindgen.py r=emilio

After cargo 7caa161, which was included in rust 1.43 nightly 2020-02-23 as part of rust 11530de, if you don't have rustc installed in the normal place (which our CI builders don't) then you need to specify RUSTC explicitly during cargo metadata or else the call will fail. In our build system, the RUSTC wasn't being passed through in RunCbindgen.py.

Differential Revision: https://phabricator.services.mozilla.com/D68086

--HG--
extra : moz-landing-system : lando

build/RunCbindgen.py

@@ -26,6 +26,7 @@ def _generate(output, cbindgen_crate_path, metadata_crate_path,
               in_tree_dependencies):
     env = os.environ.copy()
     env['CARGO'] = str(buildconfig.substs['CARGO'])
+    env['RUSTC'] = str(buildconfig.substs['RUSTC'])
 
     p = subprocess.Popen([
         buildconfig.substs['CBINDGEN'],