Remote backend incorrectly handles remote stored variables of non-string types

[apparentlymart]

Terraform Version

Terraform 0.12.12

Terraform Configuration

Any configuration using the remote backend with at least one non-string variable that is set remotely in the Terraform Cloud/Enterprise workspace settings is subject to this limitation.

terraform {
  backend "remote" {
    # (any valid remote backend configuration referring to
    # a workspace with a stored value for "example")
  }
}

variable "example" {
  type = list(string)
}

Expected Behavior

When running a Terraform command that operates only locally, such as terraform import, the remote backend should retrieved the stored variable value and parse it as HCL (due to the HCL checkbox being set) in order to set the value for the variable.

Actual Behavior

The remote backend always interprets all variable values as strings, causing the above to be interpreted as "[\"a\", \"b\"]" instead of as ["a", "b"]. Because there's a type constraint on that variable, the variable value fails validation with an error:

Error: Invalid value for input variable

The environment variable TF_VAR_example does not contain a valid value for
variable "example": list of string required.

It is additionally misreported as being set by environment variable because that is the source type used by the remote backend when populating these variables fetched from the remote workspace.

Steps to Reproduce

With a configuration and remote workspace configuration like the above, run a command like terraform import which attempts to construct a local context for their operation after fetching the variables from the remote workspace.

Additional Context

The relevant code that is assuming all variable values are strings is here:

terraform/backend/remote/testing.go

Lines 286 to 289 in ce68b4d

	return &terraform.InputValue{
	Value: cty.StringVal(v.value),
	SourceType: v.source,
	}, tfdiags.Diagnostics{}

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