ci: add missing xtensa-amd_acp_7_3 toolchain and enable provenance attestation

[beriberikix]

Summary

Fix workflow-level warnings and metadata issues seen in GitHub Actions annotations for container builds.

Changes

W4 — Split collapsed xtensa-amd_acp entry into two toolchains

In the GNU matrix, one entry incorrectly combined:

• arch: xtensa-amd_acp_7_0_adsp_zephyr-elf
• nick: xtensa-amd_acp_7_3_adsp

This effectively dropped the 7.3 toolchain and mislabeled the 7.0 image tag. Replaced with two explicit entries:

• xtensa-amd_acp_7_0_adsp_zephyr-elf / xtensa-amd_acp_7_0_adsp
• xtensa-amd_acp_7_3_adsp_zephyr-elf / xtensa-amd_acp_7_3_adsp

W5 — Enable provenance attestations

For all four jobs (base, gnu, llvm, posix):

• provenance: false -> provenance: true
• Added required job permissions:
  • attestations: write
  • id-token: write

This resolves the build-push-action provenance warning while preserving existing build/push behavior.

Risk

Low. Changes are limited to workflow metadata and matrix entries; no Dockerfile or runtime logic changes.