Preserve file permissions in artifacts

Uploading and downloading artifacts doesn't properly preserve file modes. Downloading executables in particular would be very useful since when moving tasks out into concurrent steps it can break build scripts to lose the executable bit. Also not great when building docker images out of the executables.

https://buildkite.com/betterteamapp/dummy-pipeline/builds/7#2db28ceb-5fd4-45db-abe9-8dbb4f1623cd is a repro of the issue. It's a dummy repro, but the key is the pipeline:

``` yaml
steps:
- command: "echo \"#/usr/bin/env bash\\necho wow\" > wow\nchmod +x wow\nbuildkite-agent artifact upload wow"

- wait

- command: "buildkite-agent artifact download \"wow\" .\nls -la\n./wow"
```

Expected: `./wow` outputs `wow`.
Actual: Build fails.