[rust] Fix upstream "main" -> "master"

upstream uses "master".

Change-Id: I389a2fd263e601877e996f6559b9b994fac43d19
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4496865
Reviewed-by: danakj <danakj@chromium.org>
Auto-Submit: Arthur Eubanks <aeubanks@google.com>
Commit-Queue: Arthur Eubanks <aeubanks@google.com>
Cr-Commit-Position: refs/heads/main@{#1141700}
NOKEYCHECK=True
GitOrigin-RevId: f1d3b193be6a1f58367b0acfd44ecdd4a3cf3218

Author: aeubanks

build_rust.py

@@ -537,11 +537,12 @@ def MakeVersionStamp(git_hash):
 
 def GetLatestRustCommit():
     """Get the latest commit hash in the LLVM monorepo."""
+    url = (
+        'https://chromium.googlesource.com/external/' +
+        'github.com/rust-lang/rust/+/refs/heads/master?format=JSON'  # nocheck
+    )
     main = json.loads(
-        urllib.request.urlopen('https://chromium.googlesource.com/external/' +
-                               'github.com/rust-lang/rust/' +
-                               '+/refs/heads/main?format=JSON').read().decode(
-                                   "utf-8").replace(")]}'", ""))
+        urllib.request.urlopen(url).read().decode("utf-8").replace(")]}'", ""))
     return main['commit']