Use standard Scaleway env vars / config file

[ghost]

This issue was originally opened by @OJFord as hashicorp/packer#8191. It was migrated here as a result of the Packer plugin split. The original body of the issue is below.

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Feature Description

Terraform uses the same environment variables and config files as used by Scaleway's own tooling, unless overridden:
https://www.terraform.io/docs/providers/scaleway/index.html#environment-variables

Packer however uses different environment variables (that sound 'official' enough, but are actually just for packer) and ignores the config file:
https://www.packer.io/docs/builders/scaleway.html#required-

Use Case(s)

Harmony among tools!

[Codelax]

As specified in doc, env variables work.
Config also work, if you have a working one a do not put secrets in your packer file, it will use your default profile.
You can also choose your profile with SCW_PROFILE