feat: add ALLOW_CROSS_PLATFORM_IMAGES param to multi-platform pipeline

[sampras343]

Summary

• Expose ALLOW_CROSS_PLATFORM_IMAGES parameter in docker-build-multi-platform-oci-ta.yaml, matching existing support in docker-build-oci-ta.yaml and bundle-build-oci-ta.yaml
• The buildah-remote-oci-ta task already reads this env var but the multi-platform pipeline did not expose it — defaults to "false" so all existing builds are unaffected

Why

The inverted cli-stack build flow (securesign/timestamp-authority#391) has a multi-platform component image that extracts binaries from a single-arch cli-stack image via the wait-for-pipelinerun mechanism. Non-amd64 builders reject the amd64 cli-stack image without ALLOW_CROSS_PLATFORM_IMAGES=true.

Test plan

• Existing builds (cosign, gitsign, rekor, etc.) continue to work — they don't set this param, so default "false" applies
• timestamp-authority inverted flow PR (#391) passes all platform builds with ALLOW_CROSS_PLATFORM_IMAGES: "true"

[github-actions]

Configuration Diff

1 document(s) impacted:

+ 0 added
- 0 removed
! 1 modified

Diff

@@ spec.resources.appstudio.redhat.com/v1alpha1/Component/fetch-tsa-certs{{.nameSuffix}}.spec @@
# projctl.konflux.dev/v1beta1/ProjectDevelopmentStreamTemplate/rhtas-tenant/tas-tools-template
! - one map entry removed:
- build-nudges-ref:
- - fetch-tsa-certs-cli-stack{{.nameSuffix}}

---

_{📦 Artifacts: base-output.yaml, head-output.yaml, dyff-output.txt}