Bug 1494387 - Update windows builders to rustc 1.30 beta. r=froydnj

This updates our windows builders to use a version of rustc that includes
https://github.com/rust-lang/rust/pull/52847 We're not commiting to letting
this change ride the trains at this time.

Differential Revision: https://phabricator.services.mozilla.com/D7663

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

taskcluster/ci/toolchain/windows.yml

@@ -159,7 +159,8 @@ win64-rust-1.29:
         using: toolchain-script
         script: repack_rust.py
         arguments: [
-            '--channel', '1.29.0',
+            # Rustc 1.30 beta for rust-lang/rust#52847
+            '--channel', 'beta-2018-09-28',
             '--host', 'x86_64-pc-windows-msvc',
             '--target', 'x86_64-pc-windows-msvc',
             '--target', 'i686-pc-windows-msvc',