'brave base' command fails on Windows when exporting image

[Szubie]

Describe the bug
brave base command fails on Windows on final step, when exporting image.

To Reproduce
Run brave base github.com/beringresearch/bravefiles/ubuntu/ubuntu-bionic-py3 on Windows.

After all dependencies are installed using apt, on the final step, the build will fail.

Expected behavior
Expected brave image to be built successfully.

Command line log
Publishing image ubuntu-bionic-py3
Exporting image ubuntu-bionic-py3
Failed to export image: remove ubuntu-bionic-py3-1.0.root: The process cannot access the file because it is being used by another process.

Environment (please complete the following information):

• Your operating system name and version: Windows 10 Pro version 1903 (OS Build 18362.1082) 64-bit
• Bravetools version: 1.53
• LXD version: 4.4

[idroz]

This was merged in f172c8b and included in #16

If issue persists, do reopen.