Fall back to last working Rust build

A recent Clang roll did not produce a working Rust package. There are
two bugs blocking a Rust roll to fix the issue. In the meantime, fall
back to the last package.

Additionally, this changes update_rust.py to handle when the version stamp in the existing Rust package does not have the expected format. In this case it will always download and unpack a new package.

Bug: 1379429, 1377429
Change-Id: I6dbbabedc710ea06d964206d4e0b6b1c6b8afac4

Cq-Include-Trybots: luci.chromium.try:android-rust-arm-dbg,android-rust-arm-rel,linux-rust-x64-dbg,linux-rust-x64-rel
Change-Id: I6dbbabedc710ea06d964206d4e0b6b1c6b8afac4
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3999834
Reviewed-by: Nico Weber <thakis@chromium.org>
Commit-Queue: Collin Baker <collinbaker@chromium.org>
Commit-Queue: Nico Weber <thakis@chromium.org>
Auto-Submit: Collin Baker <collinbaker@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1066727}
NOKEYCHECK=True
GitOrigin-RevId: 152639d5b6935f508051ffd456a3225de5dbdf86

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 = None
+FALLBACK_CLANG_VERSION = 'llvmorg-16-init-8697-g60809cd2-1'
 
 # 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
@@ -91,7 +91,10 @@ def GetStampVersion():
         with open(VERSION_STAMP_PATH) as version_file:
             existing_stamp = version_file.readline().rstrip()
         version_re = re.compile(r'rustc [0-9.]+-dev \((.+?) chromium\)')
-        return version_re.fullmatch(existing_stamp).group(1)
+        match = version_re.fullmatch(existing_stamp)
+        if match is None:
+            return None
+        return match.group(1)
 
     return None