Reusable workflow improvements in Depot CI

We've shipped a batch of improvements to reusable workflow support in Depot CI:

• strategy.matrix in reusable workflows — Matrix expansion now works correctly inside workflow_call reusable workflows.
• Composite action environment propagation — GITHUB_ENV writes inside composite actions now propagate to subsequent job steps.
• Nested secret forwarding — Secrets are correctly forwarded through nested reusable workflow chains, so secrets: inherit works at every level.
• Fan-in with if: always() — Jobs with if: always() now correctly run even when some upstream needs jobs are skipped.
• Dynamic matrix in reusable workflows — Dynamic matrix expansion via fromJSON() now works inside reusable workflows called via workflow_call.