terraform/exec: Pass through all environment variables

[radeksimko]

Closes #138

This should address many use cases where some form of external auth provider is being looked up by the provider or state backend.
e.g. https://docs.aws.amazon.com/eks/latest/userguide/install-aws-iam-authenticator.html in case of Kubernetes or az (Azure CLI) in case of AzureRM.

and also allow authentication from behind a corporate proxy, which can be configured on the workstation in such a way that the address of the proxy server is in standard environment variables.

We pass through all variables that are given to the server, which means that these could be reused between different workspaces, but this should be fairly safe for now, because

a) the language server doesn't execute terraform command that could do damage when provided wrong variables. We are working with the assumption that terraform providers schema -json does not push state.
b) we only allow one workspace at a time to be initialized anyway

[ghost]

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