Add ARM64 miniconda testbed using modern directives

[Vbitz]

Summary

• add a reusable modern-directive include for Miniconda at examples/includes/modern/miniconda.yaml
• add examples/recipes/test-arm64-miniconda/build.yaml as a template-free ARM64/x86_64 testbed using variables.try + arch conditions (qsmxt style)
• include existing builder build memory flag update (-m 12g) in cmd/builder/main.go

Why

• avoids reliance on legacy neurodocker template YAML for architecture-specific installer selection
• provides a generalized pattern that can be copied into modern directive-based recipes

Validation

• go run ./cmd/builder generate --config examples/config.yaml test-arm64-miniconda
• go run ./cmd/builder build --config examples/config.yaml test-arm64-miniconda (built successfully on ARM64)