Vendir should preserve execution permissions on extract

**What steps did you take:**

```yaml
- path: terraform.d/plugins
contents:
- path: carvel.dev/carvel/k14s/0.6.0
githubRelease:
slug: vmware-tanzu/terraform-provider-carvel
tag: v0.6.0
assetNames: ["terraform-provider-k14s-binaries.tgz"]
unpackArchive:
path: terraform-provider-k14s-binaries.tgz
```

`vendir sync`

**What happened:**
The extracted package binaries will be set as non-executable even though in the archive they are executable.

**What did you expect:**
The file permissions should be left alone and they should be executable.

**Anything else you would like to add:**
This is very important for downloading of CLI tools

**Environment:**

- vendir version (execute `vendir --version`): 0.13.0
- OS (e.g. from `/etc/os-release`): Ubuntu 20.04