Bug 1506493 - Update Rust nightly version for Mac cross-compilation. r=me

DONTBUILD as the code coverage build is mozilla-central only

--HG--
extra : rebase_source : 94d75e99dde2938150d0ca867fb706963c856ef2
extra : histedit_source : 8284f911201465cfe71b26c75bf048f47b8710f7

taskcluster/ci/toolchain/linux.yml

@@ -575,7 +575,7 @@ linux64-rust-nightly-macos:
         using: toolchain-script
         script: repack_rust.py
         arguments: [
-            '--channel', 'nightly-2018-09-16',
+            '--channel', 'nightly-2018-10-05',
             '--host', 'x86_64-unknown-linux-gnu',
             '--target', 'x86_64-unknown-linux-gnu',
             '--target', 'x86_64-apple-darwin',