use the cached Terraform providers if available

[glennmusa]

Description

Assigns the $TF_PLUGIN_CACHE_DIR to the plugin-dir=PATH terraform init option so that, in a disconnected state, the Terraform providers that are bundled by container build are used instead of querying out to the internet for them and/or their metadata.

If the $TF_PLUGIN_CACHE_DIR is not present on the environment executing Terraform, nothing changes.

Issue reference

The issue this PR will close: #275

Checklist

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