[pytorch][ci] add build_only flag to mobile CI jobs

[ljk53]

Stack from ghstack:

• [pytorch][ci] add build_only flag to mobile CI jobs #34560 [pytorch][ci] add build_only flag to mobile CI jobs

Summary:
These jobs don't have next phase so we don't really need commit the
docker images.
Should also fix issue #34557.

Differential Revision: D20375308

[dr-ci]

💊 CircleCI build failures summary and remediations

As of commit 3f694c3 (more details on the Dr. CI page):

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✅ None of the build failures appear to be your fault 💚

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• 1/1 broken upstream at merge base 259d729 since Mar 10

  Please rebase on the viable/strict branch (expand for instructions)

  Since your merge base is older than viable/strict, run these commands:

  git fetch origin viable/strict
  git rebase viable/strict
  

  Check out the recency history of this "viable master" tracking branch.

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🚧 1 upstream failure:

These were probably caused by upstream breakages:

• pytorch_linux_xenial_py3_6_gcc5_4_build from Mar 10 until Mar 10

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@ljk53 merged this pull request in 4167db1.