Disable Clang version fallback for Rust package

Bug: None
Change-Id: I5798f77d8cec0643a311ae3e4992c127f144555f

Cq-Include-Trybots: luci.chromium.try:android-rust-arm-dbg,android-rust-arm-rel,linux-rust-x64-dbg,linux-rust-x64-rel
Change-Id: I5798f77d8cec0643a311ae3e4992c127f144555f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3868496
Reviewed-by: Łukasz Anforowicz <[email protected]>
Commit-Queue: Collin Baker <[email protected]>
Cr-Commit-Position: refs/heads/main@{#1042291}
NOKEYCHECK=True
GitOrigin-RevId: f1f59c7d295a05f24f52e94835053f00077e17a8

Author: chbaker0

update_rust.py

@@ -45,7 +45,7 @@
 # This should almost always be None. When a breakage happens the fallback should
 # be temporary. Once fixed, the applicable revision(s) above should be updated
 # and FALLBACK_CLANG_VERSION should be reset to None.
-FALLBACK_CLANG_VERSION = 'llvmorg-16-init-907-g8b740747-4'
+FALLBACK_CLANG_VERSION = None
 
 # Hash of src/stage0.json, which itself contains the stage0 toolchain hashes.
 # We trust the Rust build system checks, but to ensure it is not tampered with