Enable ability for global environment variables across all tasks

[jcafarelli-dotmatics]

This is kind of a feature request that is also described in https://github.com/OctopusDeploy/Issues/issues/3712.

It would be nice if there was a way to set environment variables for all tasks so we didnt have to manually set them by grabbing them from octopus variables with get_octopusvariable. This would enable us to set account secrets for terraform runs by using octopus accounts and variables at a global level like variable sets, without also having to change every single terraform project.

one way I was thinking this could be easily achieved and make use very simple, is just put Environment Variable as a type when creating a variable in a variable set in the dropdown shown below. This way people could choose what to expose as an env var, and in the backend code for octopus just pull those types and export them before running any tasks.

This approach would make any bash scripts run easier locally as well as in octopus; and really help when trying to troubleshoot deployment problems.