ci: Add update-nixpkgs tooling

[leona-ya]

@flyingcircusio/release-managers

PL-133100

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)
  • Should work, should not interfere with current release branches in fc-nixos and nixpkgs
• Security requirements tested? (EVIDENCE)
  • Tested in fc-nixos-testing
  • By design the scripts only automatically update integrations branches
  • The actual review/merge is done by a human and assisted by GHA