Investigate Sketch export including the rounded corners only when exported from cli

[ralfebert]

Sketch bugreport:

Summary (as much information as possible):
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cli sketchtool export output differs from export output from the Sketch app.

Steps to reproduce:
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Use the iOS App Icon Template and Export the slices from the Sketch App and using the command line tool with:
'/Applications/Sketch.app/Contents/MacOS/sketchtool export slices AppIcon.sketch'
as described in https://developer.sketch.com/cli/export-assets

Expected results:
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Both sets of exported files should not have rounded corners because they are not on the layer that's set up for exporting.

Actual results:
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Export works correctly when exported via Sketch app, exported with rounded corners when exported using the command line.

- Sketch version: 76.1 
- macOS version: 11.5.2

[ralfebert]

Example files to reproduce:
https://github.com/ralfebert/AppIcon-SketchExport