allow gs_downloader to use GS_APPLICATION_CREDENTIALS

[GaryPWhite]

Behind our firewall, the default google client will explode when performing some operations. This PR is to address that issue by allow us to authenticate during the download process with the client. By not using the default client, we're able to download protected artifacts and bypass the firewall restriction.

Let me know if we have any problems/questions, and thanks!

[lox]

Is newGoogleClient defined somewhere else?

[GaryPWhite]

It's defined in gs_uploader. Anything you think I ought to do?

[GaryPWhite]

Bumping in case y'all have more questions or if I need to contribute elsewhere to make this work 😁

[lox]

For future generations, the function this change references is at

agent/agent/gs_uploader.go

Line 74 in 6be67b3

func newGoogleClient(scope string) (*http.Client, error) {

I reckon in future we should avoid new* on constructors that have extra behaviour like reading env.

[lox]

Of course I wrote that code. 🤦🏼‍♂️

[lox]

Sorry that took me so long @GaryPWhite. Gotta get us some more Golang developers.

[GaryPWhite]

Hiring? ;)

[harrietgrace]

Docs updated: buildkite/docs#556