Skip rolling Rust for now, reset versions to the last successful build

When we try roll Clang + Rust and it fails, the versions are still
being changed. This is fine for the toolchain as we have a
FALLBACK_REVISION pinned to the last successful build of Rust.

However for pulling rust_src to build stdlib on dev machines, it's not
fine as the version comes from DEPS and isn't based on the
FALLBACK_REVISION. This is now blocking Collin's work on getting the
stdlib build up and running.

We will resolve this by:
a) Pulling Rust sources from git when building the toolchain
b) Packaging the stdlib sources from there so they match the
   toolchain build, and shipping those to devs.
c) Pulling the stdlib sources with the same mechanism as the toolchain
   (through update_rust.py) so they both use the FALLBACK_REVISION.

[email protected]

Bug: 1401042
Change-Id: I1dc0cf29852691842adf2ae40f16841a6717f591
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4191409
Reviewed-by: Hans Wennborg <[email protected]>
Commit-Queue: danakj <[email protected]>
Cr-Commit-Position: refs/heads/main@{#1096744}
NOKEYCHECK=True
GitOrigin-RevId: 5bd272bc07b0f65b5f3f5b24fc2316aed0b24ac6

Author: danakj

update_rust.py

@@ -30,7 +30,7 @@
 # These fields are written by //tools/clang/scripts/upload_revision.py, and
 # should not be changed manually.
 RUST_REVISION_TAG = '2'
-RUST_REVISION = '20230117'
+RUST_REVISION = '20221209'
 RUST_SUB_REVISION = 1
 
 # Trunk on 2022-10-15.