Support for private Hex packages

If a private hex package is being used (documented here https://hex.pm/docs/private), the deployment currently fails. Seems like there needs to be a way to pass in a hash and then call `mix hex.organization auth acme --key THE_HASH` if a key is specified somewhere, maybe in the app.yaml file?

```
Step #1: Failed to fetch record for 'hexpm:foo/foo' from registry (using cache instead)
Step #1: This could be because the package does not exist, it was spelled incorrectly or you don't have permissions to it
Step #1: ** (Mix) Unknown package foo in lockfile
Step #1: The command '/bin/sh -c mix do deps.get, deps.compile, compile' returned a non-zero code: 1
Finished Step #1
ERROR
ERROR: build step 1 "gcr.io/cloud-builders/docker:latest" failed: exit status 1
```