Management script does `cd "$RELEASE_ROOT_DIR"`

### Steps to reproduce

```
$ MIX_ENV=prod mix release

$ cat > script.ex <<\EOF
IO.puts("hello from script.ex")
EOF

$ ./_build/prod/rel/myapp/bin/myapp eval --file ./script.ex
▸ Could not load /home/hrubi/tmp/myapp/_build/prod/rel/myapp/script.ex: could not load /home/hrubi/tmp/myapp/_build/prod/rel/myapp/script.ex
▸ (elixir) lib/code.ex:1147: Code.find_file/2
▸ (elixir) lib/code.ex:706: Code.eval_file/2
▸ (distillery) lib/mix/lib/releases/runtime/control.ex:704: Mix.Releases.Runtime.Control.eval/2
▸ (distillery) lib/entry.ex:44: Mix.Releases.Runtime.Control.main/1
▸ (stdlib) erl_eval.erl:680: :erl_eval.do_apply/6
▸ (elixir) src/elixir.erl:258: :elixir.eval_forms/4
▸ (elixir) lib/code.ex:232: Code.eval_string/3
▸ (elixir) lib/kernel/cli.ex:503: Kernel.CLI.wrapper/1
```

### Description of issue

I found it inconvenient that when running the release management script commands which take a path as an argument (e.g. the `eval` or a custom one), that the path is interpreted as relative to the `RELEASE_ROOT_DIR`. It is counter-intuitive for the end user of the management script commands as he has to know that the relative path is treated that way or to use absolute path.

I'm not yet sure about the motivation why the `_build/prod/rel//releases//myapp.sh` does cd into the release root nor what are the implications of not doing so. The source code in question is in [priv/templates/release_rc_main.eex](https://github.com/bitwalker/distillery/blob/4ebfe9a6464d981b77008e083d258b4b720c6838/priv/templates/release_rc_main.eex#L89)

Distillery: 2.0.14
Erlang: 21.1.1
OS: Linux