Allow bootstrap extensions to download/install correct ARM vs. Intel extension on macOS

[juliasilge]

Related to:

• Consider switching ruff formatter in lieu of black formatter #6461
• Add air extension as "bootstrapped" extension in Positron #6462

Each of these extensions provides a binary, and there are separate extension versions for ARM vs. Intel architectures in addition to separate versions per OS. We currently make separate ARM vs. Intel builds for Linux so we don't have a problem there, but we make a universal build on macOS. We need for the code to bootstrap/install extensions to account for the architecture on macOS.