Currently pkg/archive stores nested windows files with
backslashes (e.g. `dir\`, `dir\file.txt`) and this causes
tar not being correctly extracted on Linux daemon.
This change assures we canonicalize all paths to unix
paths and add them to tar with that name independent of platform.
Fixes the following test cases for Windows CI:
- TestBuildAddFileWithWhitespace
- TestBuildCopyFileWithWhitespace
- TestBuildAddDirContentToRoot
- TestBuildAddDirContentToExistingDir
- TestBuildCopyDirContentToRoot
- TestBuildCopyDirContentToExistDir
- TestBuildDockerignore
- TestBuildEnvUsage
- TestBuildEnvUsage2
Signed-off-by: Ahmet Alp Balkan <ahmetalpbalkan@gmail.com>
