This reverts commit 688be43811007d33b9c70dec93b43782a4f71c06; instead
of copying the ISO (which can be huge), we have adjusted the cache path
to include the extension of the source file.
Conflicts:
builder/virtualbox/step_download_iso.go
When verifying large files, the checksum would take a long time and
downloader would be nil. When the goroutine asked for progress,
it would raise a nil exception.
Emil Sit
Mitchell Hashimoto