add tracing for when evaluation recurses into 21.05 nixpkgs instance

[osnyx]

When the evaluation traverses into the nixpkgs 21.05 revision we still ship for certain hardware and infrastructure features, issue a trace message. This helps debugging the eval depth and discover cases where that path is evaluated unexpectedly, we don't expect that to be the case on virtual machines.

part of PL-133522

@flyingcircusio/release-managers

Release process

• Created changelog entry using ./changelog.sh

PR release workflow (internal)

• PR has internal ticket
• internal issue ID (PL-…) part of branch name
• internal issue ID mentioned in PR description text
• ticket is on Platform agile board
• ticket state set to Pull request ready
• if ticket is more urgent than within the next few days, directly contact a member of the Platform team

Design notes

• Provide a feature toggle if the change might need to be adjusted/reverted quickly depending on context. Consider whether the default should be on or off. Example: rate limiting.
• All customer-facing features and (NixOS) options need to be discoverable from documentation. Add or update relevant documentation such that hosted and guided customers can understand it as well.

Security implications

• Security requirements defined? (WHERE)
  • availability: provide tracing for a potential cause of higher memory consumption of system evalutaions
  • introduce no know regressions
• Security requirements tested? (EVIDENCE)
  • automated tests still pass
  • tested on physical infrastructure host and test VM that traces appear as expected

[github-actions]

This PR is ready to merge. Merge scheduled for 2025-03-13