Fix mb train for phased builders.

Currently, when producing expectations for phased builders, mb train
puts the GN args as the keys directly within the phase's dict. In
contrast, non-phased builders GN args appear under the gn_args key for
the builder and an args_file key will be generated for builders that
import other files. This change unifies the mb train behavior to do the
same for the phases of a phased builder.

Bug: 1304842
Change-Id: I4e5f42ed5f9510ff4ad083360ef8a79f46d99b8d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3513521
Reviewed-by: Ben Pastene <bpastene@chromium.org>
Commit-Queue: Garrett Beaty <gbeaty@google.com>
Cr-Commit-Position: refs/heads/main@{#979439}