Variant environment variables take precedence over environment variables of a package  #186

Bug description

When you use variants with leeway and specify and environment variable and have a package which also specifies the same environment variable, the variant's setting will be preferred.

I believe this to be a bug because a package is more specific than a variant (although they're not directly related) and in our case in https://github.com/gitpod-io/gitpod it let to unexpected behavior with a global setting for GOARCH and our inability to override it inside a package building our CLI for different platforms.

Steps to reproduce

WORKSPACE.yaml:

defaultVariant:
  env:
    - GOARCH=amd64

BUILD.yaml:

packages:
  - name: env-test
    type: generic
    env:
      - GOARCH=arm64
      - GOOS=darwin
    config:
      commands:
        - ["sh", "-c", "echo $GOARCH $GOOS && exit 1"]

Expected behavior

Building the above results in arm64 darwin instead of the actual amd64 darwin.

Example repository

No response

Anything else?

It may not be clear-cut what of the two should actually have preference over the other and if it's the case there's no common agreement, I can also see a possibility for a leeway package to specify its own default variant or other opt-out of global settings.