feat: build environment variables

[shekhirin]

Google allows to update Build Environment Variables on deploy, so let's make use of such opportunity: https://cloud.google.com/functions/docs/env-var#updating_build_environment_variables.

buildEnvironmentVariables property is from REST API documentation https://cloud.google.com/functions/docs/reference/rest/v1/projects.locations.functions

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Now we have a better solution for private Go dependencies than having a vendor. With build env vars, you can set GOPROXY and buildpack will fetch needed private dependencies through it. Yay.

[shekhirin]

Hi @medikoo! Could you please review the PR when you have a time?

[pgrzesik]

Hello @shekhirin - sorry for not getting into your Pr sooner - could you please rebase it on top of current master and request a review from me? Thanks :bow:

[shekhirin]

Hey @pgrzesik, just rebased! Seems like I don't have access to requesting a review 🤷‍♂️

[and3rson]

Just found this while looking for a way to set GOOGLE_FUNCTION_SOURCE which is critical for our deployment process and is the only thing blocking us from moving to GCP.

Any chance of getting this merged anytime soon? Thanks!

[PrimeDominus]

@shekhirin Maybe you need to rebase this to resolve conflicts?