virtiofs: disable global cache policy

[kajusnau]

Description of Changes

Disabled global cache=always policy to fix coherency issues

Let's re-enable the cache for /nix/store shares only, when/if microvm-nix/microvm.nix#487 is merged and available.

Type of Change

• New Feature
• Bug Fix
• Improvement / Refactor

Related Issues / Tickets

Fixes https://jira.tii.ae/browse/SSRCSP-8196

Checklist

• Clear summary in PR description
• Detailed and meaningful commit message(s)
• Commits are logically organized and squashed if appropriate
• Contribution guidelines followed
• Ghaf documentation updated with the commit - https://tiiuae.github.io/ghaf/
• Author has run make-checks and it passes
• All automatic GitHub Action checks pass - see actions
• Author has added reviewers and removed PR draft status

Testing Instructions

Applicable Targets

• Orin AGX aarch64
• Orin NX aarch64
• Lenovo X1 x86_64
• Dell Latitude x86_64
• System 76 x86_64

Installation Method

• Requires full re-installation
• Can be updated with nixos-rebuild ... switch
• Other:

Test Steps To Verify:

1. Verify the bug is fixed

[milva-unikie]

Tested on Darter Pro

• https://jira.tii.ae/browse/SSRCSP-8196 has been fixed