Set User-Agent header when fetching TF or any other download

[paultyng]

Also needs to set TF_APPEND_USER_AGENT on invocations of terraform.

[bflad]

terraform-exec automatically propagates TF_APPEND_USER_AGENT: https://github.com/hashicorp/terraform-exec/blob/40a11613e126074b31cfc07c787de74dee0a923e/tfexec/cmd.go#L137-L143

If desired, a separate feature request can be submitted to hc-install to allow explicitly setting the User-Agent, which when in place we can then implement here by passing through TF_APPEND_USER_AGENT. 👍

[github-actions]

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