fix PostCommit Python Arm SDK container

[aIbrahiim]

Fixes PostCommit Python Arm misconfiguration mentioned on #39653.

The job was submitting an x86 wheel from checkout while workers used Snapshots :latest
on the release branch runs that showed up as 2.76 vs 2.77

-Pin the container image to checkout sdk_version instead of :latest
-Use sdk_location=container with prebuilt Snapshots images (drop x86 wheel staging)

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