Backport of 'Allow security policy bot cross-project container registry access' to 19.4

What does this MR do and why?

Backports !257932 (merged) to 19.4.

The security policy bot only gets its permissions on projects where it is a member. Scan execution policy jobs therefore cannot pull scanner images from another project's private container registry, even when that project allows the bot's project in its CI/CD job token allowlist.

The fix adds a security_policy_bot_via_job_token condition that grants only build_read_container_image when the bot authenticates with a CI job token and the target project's inbound job token scope allows the source project.

This is a clean cherry-pick of af9b30a5 with no conflicts.

References

• !257932 (merged)
• #630692

MR acceptance checklist

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• This MR is backporting a bug fix, documentation update, or spec fix, previously merged in the default branch.
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• The MR title is descriptive (e.g. "Backport of 'title of default branch MR'"). This is important, since the title will be copied to the patch blog post.
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