Depot CI workflow condition and expression fixes

We fixed several GitHub Actions compatibility differences in Depot CI workflow evaluation:

• Steps with an ordinary if: condition now respect GitHub's implicit success() check.
• Reusable workflow calls remain successful when some inner jobs are skipped and the remaining jobs succeed.
• Dynamic job environment expressions now select the correct environment-scoped secrets and variables.
• Missing event properties passed to composite action inputs now resolve to an empty value instead of literal expression text.