Add support for environment variables with pack build

[aranw]

This PR adds support for environment variables with the pack build phase

Some buildpacks allow configuration via environment variables such as https://github.com/GoogleCloudPlatform/buildpacks see https://github.com/GoogleCloudPlatform/buildpacks#common-options for available environment variables

I copied the configuration style from the docker build stage so configuration looks like the following:

    build {
        use "pack" {
            builder     = "gcr.io/buildpacks/builder"
            static_environment = {
                "GOOGLE_BUILDABLE" : "./cmd/webhook"
            }
        }
    }

I've tested it locally and can see from the output that it used my environment variable as expected

 │ [builder] Running "go build -o /layers/google.go.build/bin/main ./cmd/webhook (GOCACHE=/layers/google.go.build/gocache)"

[hashicorp-cla]

All committers have signed the CLA.

[mitchellh]

This looks great thank you!