Add prod_envs option for custom production Mix environments

[johns10]

Summary

• Add a prod_envs option to the burrito release config that controls which Mix environments are treated as production builds
• Defaults to [:prod] for full backward compatibility
• Fixes builds for projects using custom production environments like :prod_cli

Problem

is_prod/1 in PackAndBuild is hardcoded to check Mix.env() == :prod. Projects that use custom production environments (e.g. :prod_cli for building a CLI binary alongside a web app from the same codebase) always get IS_PROD=0, causing the Zig launcher to skip payload extraction and the release to silently fail to boot.

This is common when the same Elixir project needs different compile-time configs for different deployment targets (e.g. SQLite for CLI vs Postgres for web).

Solution

# In release config:
burrito: [
  prod_envs: [:prod, :prod_cli],
  targets: [...]
]

The prod_envs option accepts a list of atoms. When Mix.env() matches any entry, IS_PROD is set to "1". The default [:prod] preserves existing behavior.

Test plan

• Verified IS_PROD=1 in Zig build env with prod_envs: [:prod, :prod_cli]
• Verified Burrito binary extracts payload and boots successfully
• Verified default behavior unchanged (:prod still works without the option)

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