Improve support for running in read-only environments

### Steps to reproduce

* Use Distillery 2.0.13 or 2.0.14
* Deploy to read-only filesystem (embedded linux box)

Try to start app with e.g. (note that `/data` is read-write)

```
HOME=/data/balrog/ RELEASE_MUTABLE_DIR=/data/balrog/ /balrog/bin/balrog foreground
```

### Verbose Logs (Logs from embedded box)

```
sh: /balrog/releases/0.2.3/libexec/erts.sh: line 306: /balrog/releases/start_erl.data: Read-only file system
```

### Description of issue

This works fine on Distillery 2.0.12, but fails with the `Read-only filesystem` message on 2.0.13 and 2.0.14.

The app is deployed to a read-only dir, but I point to a read-write dir for the `$RELEASE_MUTABLE_DIR`.

Downgrading to Distillery 2.0.12 made it work again.

Details:

- What OS, Erlang/Elixir versions are you seeing this issue on?
Linux (embedded, ARM), Erlang 21.1, Elixir 1.8.2

- If possible, also provide your `rel/config.exs`, as it is often
my first troubleshooting question, and you'll save us both time :)

```
# rel/config.exs

# Import all plugins from `rel/plugins`
# They can then be used by adding `plugin MyPlugin` to
# either an environment, or release definition, where
# `MyPlugin` is the name of the plugin module.
~w(rel plugins *.exs)
|> Path.join()
|> Path.wildcard()
|> Enum.map(&Code.eval_file(&1))

use Mix.Releases.Config,
# This sets the default release built by `mix release`
default_release: :default,
# This sets the default environment used by `mix release`
default_environment: Mix.env()

# For a full list of config options for both releases
# and environments, visit https://hexdocs.pm/distillery/config/distillery.html

# You may define one or more environments in this file,
# an environment's settings will override those of a release
# when building in that environment, this combination of release
# and environment configuration is called a profile

environment :dev do
set cookie: :"SECRET"
end

environment :stag do
set cookie: :"SECRET"
end

environment :prod do
set cookie: :"SECRET"
end

# You may define one or more releases in this file.
# If you have not set a default release, or selected one
# when running `mix release`, the first release in the file
# will be used by default

release :balrog do
set version: current_version(:balrog)
set include_erts: false
set include_src: false
end
```

I will try to dig deeper into this tomorrow.