Pass --build to x.py when building Rustc

This ensures that on Windows we build bootstrap for the MSVC API, and
avoids trying to use a missing x86_64-w64-mingw32-gcc linker.

The windows bots were choosing MSVC correctly somehow, but not all
Windows machines do (my cloudtop does not).

This is harmless on other platforms, but passing it everywhere for
consistency.

[email protected]

Bug: 1271215
Change-Id: Ib789975a2142ac965b682d073b07c823ad78fc2d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4370520
Auto-Submit: danakj <[email protected]>
Reviewed-by: Collin Baker <[email protected]>
Commit-Queue: Collin Baker <[email protected]>
Cr-Commit-Position: refs/heads/main@{#1121867}
NOKEYCHECK=True
GitOrigin-RevId: 3ba1c192e0b9af85afa80654493e4f7d935662b2

Author: danakj

build_rust.py

@@ -698,7 +698,7 @@ def main():
     else:
         assert not rest
 
-    xpy_args = []
+    xpy_args = ['--build', host_triple]
     if args.build_mac_arm:
         xpy_args.extend(['--host', host_triple, '--target', host_triple])