build API: accept platform comma separated

[Luap99]

The docker API uses only a single arg for platform and multiple platforms are given as comma separated list.

Fixes #22071

Does this PR introduce a user-facing change?

The build API now accepts platform values comma separated to better match docker.

[openshift-ci[bot]]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: Luap99

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[baude]

LGTM

[ashley-cui]

/lgtm