Relates to #43
Our previous logic did not pass options from extra_env to envbuilder.RunCacheProbe.
This fixes the logic and adds more comprehensive tests around the overriding logic.
Future commits will refactor this logic some more.
This PR adds `env_map` to `cached_image_resource.` This consists of the computed env in map format, which can be useful for other providers that do not expect `KEY=VALUE` format.
Fixes#31
We had previously been doing the equivalent of value.String() when writing envbuilder_cached_image.env. This was incorrectly escaping newlines, potentially breaking ENVBUILDER_INIT_SCRIPT.
This PR modifies the behaviour to correctly handle string values via ValueString() instead.