feat: configure cpython source

[Sl1mb0]

Closes #152

Describe your proposed changes here.

• I've read the contributing section of the project CONTRIBUTING.md.
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[Copilot]

Pull Request Overview

This PR adds configurability for CPython source downloads by introducing environment variables that allow users to specify custom URLs for Python SDK downloads. This is useful for environments with restricted internet access, custom package repositories, or mirrors.

• Adds three new configurable environment variables with sensible defaults
• Maintains backward compatibility by defaulting to the original brettcannon/cpython-wasi-build URLs
• Refactors hardcoded URLs to use the new configurable variables

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[Copilot]

The new environment variables PYTHON_SDK_BASE_URL, PYTHON_SDK_URL, and BUILD_PYTHON_SDK_URL should be documented so users know they can configure custom download locations. Consider adding a section to guests/python/README.md under the "Build" section or to CONTRIBUTING.md explaining these configuration options and their use cases (e.g., using custom mirrors, air-gapped environments).

Suggested change

	# Configurable URLs for Python SDK downloads (with defaults)
	# Python SDK download URL configuration
	#
	# The following environment variables can be set to customize the download locations for the Python WASM SDK:
	# - PYTHON_SDK_BASE_URL: Base URL for Python SDK releases (default: official GitHub release)
	# - PYTHON_SDK_URL: Full URL for the pre-built Python SDK zip file
	# - BUILD_PYTHON_SDK_URL: Full URL for the build Python SDK zip file
	#
	# These variables are useful for:
	# - Using custom mirrors or internal artifact repositories
	# - Building in air-gapped or restricted network environments
	# - Overriding the default download source for testing or reliability
	#
	# Example usage:
	# export PYTHON_SDK_BASE_URL="https://my-mirror.local/python-wasi"
	# just build-release
	#