images.fc: fix opening with kernel 5.15

[osnyx]

We're still using kernel 5.15 in our VMs for now. Opening the XFS from the VM image had failed due to incompatible XFS features.

nrext64 is only availabe from 5.19 on and non-experimental since 6.5, so we need to explicitly disable that feature for now when writing images.

PL-132780

@flyingcircusio/release-managers

Release process

Impact: -

Changelog: -

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)
  • image file system needs to be mountable by the VM kernel at bootstrap
  • must not introduce known regressions
• Security requirements tested? (EVIDENCE)
  • automated tests still pass
  • successfully mounted the image from a running 24.05 VM
    • as bootstrapping was broken all the time so far, I refrain from explicitly testing a bootstrap from this PR built product image and will do that once it has reached dev. I'm certain to have identified, tested and corrected the underlying issue.