Have the VM name configurable

[bobsira]

https://github.com/bobsira/windows-node-image-builder/blob/d88007312065281621c57e4c668207ad9a4a1a2f/windows.auto.pkrvars.hcl#L2C1-L2C51

Have this configurable in the online pipeline and local build

[bobsira]

Status: partially implemented; keeping this issue open. Local builds support -VmName and append the unique build ID to preserve isolation. The online workflow still lacks a vm_name input and does not pass -VmName to scripts\Build-WindowsImage.ps1. Also, the script default overrides vm_name from the var-file. Remaining work: expose and wire the workflow override, clarify/default the name consistently, and preserve unique temporary VM names and canonical publication naming.