fix: correctly package elixir when building binaries

[JamesPatrickGill]

DO NOT MERGE

• [ ] Tests written and linted ℹ︎
• [ ] Documentation written ℹ︎
• [ ] Commit history is tidy ℹ︎

What this does

SHOULD fix the built binaries failing when trying to run mix tasks - doesnt yet

[admons]

Superseded by https://github.com/snyk/snyk-hex-plugin/pull/15