Figure out if we can support ExportEnvFile when running from a cached image

Review: I couldn't think of a way to do this reliably. Maybe it's an easy fix but for now I'd like to leave it like this.

_Originally posted by @mafredri in https://github.com/coder/envbuilder/pull/359#discussion_r1777515547_

Start by tracing the ExportEnvFile entry point and how cached images are handled. Determine whether reliable support is possible, identify the required behavior and tests, and consider the work done only when ExportEnvFile works correctly from a cached image or the limitation is documented.

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