[mac/arm] pull python2 binary in cipd bootstrap now that it exists

Bug: 1124119,1103236,1184768
Change-Id: Ie56611586dd6f79989fba970de20846b845b08b6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/2870586
Auto-Submit: Nico Weber <thakis@chromium.org>
Commit-Queue: Dirk Pranke <dpranke@google.com>
Reviewed-by: Dirk Pranke <dpranke@google.com>

bootstrap/manifest.txt

@@ -17,9 +17,8 @@
 
 $VerifiedPlatform windows-amd64 linux-amd64 mac-amd64
 
-# TODO(https://crbug.com/1184768): Remove `=amd64` once mac/arm64 python exists
 @Subdir python
-infra/3pp/tools/cpython/${os}-${arch=amd64} version:2@2.7.18.chromium.32
+infra/3pp/tools/cpython/${platform} version:2@2.7.18.chromium.32
 
 @Subdir python3
 infra/3pp/tools/cpython3/${platform} version:2@3.8.9.chromium.14