[windows] imgpkg does not retain file permissions

**What steps did you take:**
On Windows
```
git clone https://github.com/vmware-tanzu/carvel-imgpkg
cd carvel-imgpkg
imgpkg push -f ./test/assets/bundle_file_permissions -b some-image
imgpkg pull -b some-image -o ./extract-bundle
```

Expected ./extract-bundle/exec_file.sh
to have permissions: `-rwx------`
actual : `-rw-rw-rw-`

**What happened:**
None of the file permissions were retained

**What did you expect:**
file permissions to stay the same

**Environment:**

- imgpkg version (use `imgpkg --version`):
- Docker registry used (e.g. `Docker HUB`):
- OS (e.g. from `/etc/os-release`): Windows 10

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