feat: set download mirror location default to us

[KyleTryon]

Changes

Added a new parameter mirror to the setup command. By default this changes the download mirror to the US hosted mirror, but can be reconfigured to use the CN mirror in the event a user is on CircleCI Server or using CircleCI Runner.

Resolves

• Receiving 404 when trying to download 3.7.5 binary #23
• Running into timeout issues when downloading Serverless cli #15

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