* fix: set MagicDir to tempdir when performing cache probe
* chore: update envbuilder to b7781d8
* imgutil: get default envbuilder binary path from envbuilder options
Addresses some non-blocking comments from #44:
- Extracts some of the functions in cached_image_resource.go to separate internal packages tfutil and imgutil.
- Some other functions are extracted to helpers.go.
- Extracts non-overridable flags to a package-level variable.
- Pre-allocates some slices where possible.
- Removes some unused code and renames some existing code for readability
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.