[infra] Fix pub integration test on arm64 macs

Cq-Include-Trybots: luci.dart.try:pkg-linux-release-try,pkg-mac-release-arm64-try,pkg-linux-debug-try,pkg-mac-release-try,pkg-win-release-try
Change-Id: Ied16052c7408d211cc72706a17a4157a777cf906
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/219041
Reviewed-by: Jonas Termansen <sortie@google.com>
Commit-Queue: William Hesse <whesse@google.com>
diff --git a/tools/bots/pub_integration_test.py b/tools/bots/pub_integration_test.py
index 371ed1a..f80c554 100755
--- a/tools/bots/pub_integration_test.py
+++ b/tools/bots/pub_integration_test.py
@@ -31,7 +31,7 @@
 
     (options, args) = parser.parse_args()
 
-    arch = 'XARM64' if options.arch == 'arm64' else 'X64'
+    arch = 'ARM64' if options.arch == 'arm64' else 'X64'
     mode = ('Debug' if options.mode == 'debug' else 'Release')
 
     out_dir = 'xcodebuild' if sys.platform == 'darwin' else 'out'