This fixes a regression introduced in 6d87f19142,
causing `COPY --from` to fail if the target directory does not exist:
```
FROM mcr.microsoft.com/windows/servercore:ltsc2019 as s1
RUN echo "Hello World" > /hello
FROM mcr.microsoft.com/windows/servercore:ltsc2019
COPY --from=s1 /hello /hello/another/world
```
Would produce an error:
```
Step 4/4 : COPY --from=s1 /hello /hello/another/world
failed to copy files: mkdir \\?: The filename, directory name, or volume label syntax is incorrect.
```
The cause for this was that Go's `os.MkdirAll()` does not support/detect volume GUID paths
(`\\?\Volume{dae8d3ac-b9a1-11e9-88eb-e8554b2ba1db}\hello\another}`), and as a result
attempted to create the volume as a directory (`\\?`), causing it to fail.
This patch replaces `os.MkdirAll()` with our own `system.MkdirAll()` function, which
is capable of detecting GUID volumes.
Signed-off-by: Sebastiaan van Stijn <github@gone.nl>
5858a99267