fix(prebuild): pass DayZ Steam credentials

[MarcStdt]

Summary

• declare DayZ prebuild as requiring STEAM_USER/STEAM_PASS
• pass Steam secrets through the prebuild workflow
• inherit credential env into Docker without printing secret values

Validation

• go test ./scripts/prebuild
• ./scripts/validate_all_scrolls.sh
• go run ./scripts/prebuild --targets all-steam --list-targets
• git diff --check
• workflow YAML parse

[druid-infra]

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