Add env_file configuration option to load environment variables from a file

This PR adds a new env_file configuration option that allows loading multiple environment variables from a file, providing an alternative to specifying them individually with the existing env option.

Features

• File-based environment loading: Use env_file = "./.env" to load variables from a file
• Standard .env format support: Supports KEY=VALUE format with comments and quoted values
• Flexible configuration: Can be set globally or per-job
• Proper priority handling: Direct env variables override env_file variables
• Relative path resolution: Paths are resolved relative to the package directory
• Graceful error handling: Missing files generate warnings but don't fail the build

Usage Examples

Global environment file:

# bacon.toml
env_file = "./.env"