Support passthrough of system env for local-exec provisioner

[alewando]

Allows the environment of the parent terraform process to be passed through to commands being invoked by the local-exec provisioner. Any variables explicitly specified in an environment block take precedence over those imported from the system.

I'm open to suggestions for a better argument name than system_environment.

The specific use case I'm after is passing AWS credentials down through to a local-exec provisioner. In my workflow, those (temporary) credentials are supplied as environment variables when terraform itself is invoked, so this will allow them to pass through. A solution to #8242 or #21983 would be better for a more general use case, but those don't seem likely to be implemented any time soon.

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[alewando]

Well, it turns out that local-exec already does this! Closing this PR.

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