Gruntwork Pipelines: use OpenTofu

[iangrunt]

Hello, as a Gruntwork Pipelines user, how can I use the OpenTofu binary and control the version?

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Tracked in ticket #110826

[ZachGoldberg]

FYI - There's an internal ticket tracking this work here - https://linear.app/gruntwork/issue/DEV-114/support-opentofu

[ZachGoldberg]

pipelines-execute v3.0.0-beta1 includes a new flag TF_BINARY which can be set to either terraform or opentofu (in addition to the existing TF_VERSION parameter). By default, starting in v3.0.0 we'll use OpenTofu unless the binary is forcibly set to terraform by the calling workflow.