fix: Add VirtualBox to PATH only when running VBoxManage (closes #39830)

What does this MR do?

Move the VirtualBox PATH augmentation out of init() and into prepareVBoxManageEnv(), called just before a VBoxManage command runs. The function is idempotent, and off Windows it is a no-op because VBoxManage is already on PATH.

Closes #39830

Before and after

Before: control_windows.go added the Oracle\VirtualBox directories to PATH from init(). Every runner build blank-imports the virtualbox executor, so the PATH of jobs run by other executors, including shell, was polluted.

After: A job that never invokes VirtualBox keeps its PATH unchanged, and the directories are added only when VBoxManage is about to run.

How to validate locally

1. On Windows, run go test ./helpers/virtualbox -run TestPrepareVBoxManageEnv.
2. Confirm the test passes, including the repeated-call case that checks the PATH is not extended twice.

Acceptance checklist

• PATH is not changed at startup
• PATH is extended only before a VBoxManage command
• Repeated calls do not duplicate entries
• A Windows test covers the behaviour