[21.05] fc-luks: fix external header autodetection for multiple commands

[osnyx]

I plainly forgot to pass the headerfile as an argument, despite it being properly detected before.
This fixes the execution of fc-luks keystore test-open and fc-luks check with external headers.

PL-132958

@flyingcircusio/release-managers

Release process

Impact: -

Changelog:

• fc-luks: fix execution of several commands for devices with external header

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)
  • fix a regression in automated monitoring checks
  • must not introduce known regressions
  • do a generic test on hardware in dev
  • do a specific test on an affected host
• Security requirements tested? (EVIDENCE)
  • automated tests still pass
  • manual test on dev host patty (which has no external headers)
  • manual test on prod on an affected host with external headers