fix(vhdbuilder): increase Flatcar OS disk size

[ervcz]

What type of PR is this?

/kind bug

What this PR does / why we need it:

Flatcar nodes have almost no space left for complete provisioning. Increase OS disk size from 30GB to 40GB for Flatcar ARM64 images.

Which issue(s) this PR fixes:

Requirements:

• [ ] uses conventional commit messages
• [ ] includes documentation
• [ ] adds unit tests
• [ ] tested upgrade from previous version
• [ ] commits are GPG signed and Github marks them as verified

Special notes for your reviewer:

aks-rp counterpart: https://dev.azure.com/msazure/CloudNativeCompute/_git/aks-rp/pullrequest/14186039

Release note:

Flatcar minimum disk size increased form 30 to 40GB.