fixes darwin image creation (SDK extraction)

[alexadhy]

Description:

Fixes darwin image creation (SDK is using xz instead of bz2 compression)

Checklist:

• Lint and formatter run with no errors.
• Tests all pass.

[andydotxyz]

Maybe it should somehow be .tar.* so it supports both formats? (Assuming it changed recently and users may be downloading both?).
It would be a more complicated change but feels more complete.

[alexadhy]

cool, alright . Done.

[lucor]

Thanks @alexadhy. LGTM just a little small thing before merge it.
Starting from v1.1.x the darwin image can be also generated using fyne-cross. To achieve this the darwin/Dockerfile is embedded using the make embed task.
Do you mind to run the make embed task and commit the generated resource/darwin_dockerfile.go file ?

[lucor]

Nevermind it was already there :)