Add environment variable to only get providers from get.pulumi.com

[pierskarsenbarg]

Hello!

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Issue details

There are times when customers need to be able to download providers from a known location (for example, air-gapped networks that can only access certain areas of the internet through proxies). These might not want to give access to github releases for example.

In these situations, making get.pulumi.com the location of the first request rather than GitHub releases would be crucial.

(I know this would mean no third party providers but that's a later discussion)

Affected area/feature