fix: use x86 Python 3 binary where available

Using the x86 binary is required for testing on Windows on Arm systems,
which can only emulate the x86 architecture at this time.

Change-Id: I28f01e3974b69ced4a96d8675adc024555791131
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/2108724
Reviewed-by: Robbie Iannucci <iannucci@chromium.org>
Reviewed-by: Edward Lesmes <ehmaldonado@chromium.org>
Commit-Queue: Richard Townsend <richard.townsend@arm.com>

bootstrap/manifest.txt

@@ -21,7 +21,7 @@ $VerifiedPlatform windows-386 windows-amd64 linux-amd64 mac-amd64
 infra/3pp/tools/cpython/${platform} version:2.7.17.chromium.22
 
 @Subdir python3
-infra/3pp/tools/cpython3/${os}-${arch=amd64} version:3.8.0.chromium.8
+infra/3pp/tools/cpython3/${os}-${arch} version:3.8.0.chromium.8
 
 @Subdir git
 infra/3pp/tools/git/${os}-${arch} version:2.24.1.chromium.5