fix(cs2): harden postinstall password generation

[MarcStdt]

Summary

• make CS2 postinstall password generation portable and deterministic in length
• avoid pipefail/SIGPIPE from tr | head causing silent install-5 failure

Validation

• local postinstall smoke test creates a 50-char RCON password and writes server.cfg
• go run ./scripts/validate-scrolls.go
• git diff --check

Prod finding

Fresh CS2 deployment 80e5f9f7-598f-405c-b308-c97de67bc163 reached install-5 with the concrete script but failed with empty logs, consistent with shell pipefail abort during password generation.

DayZ remains skipped per current sweep.

[druid-infra]

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