macOS-bundler: unmount old dmg when building

[lucaheise]

When (re)building the binarys the Build-Script will fail if the previouly build .dmg file is still mounted.

Steps to reproduce the behavior:

1. Build the binarys
2. Open the .dmg file
3. Close the file (Closing won't actually unmount it)
4. Build again

Expected behavior
I think there should be an exception preventing this.
Either it should check if previously the created .dmg is still mounted and exit with corresponding error-message or (better) it should try to unmount it.

While building the script will actually mount the new .dmg file and unmount it again. So unmounting it with the same process as in tauri/tooling/bundler/src/bundle/templates/dmg/bundle_dmg Line 360 should be possible as long as names dont change.

# unmount
echo "Unmounting disk image..."
hdiutil detach "${DEV_NAME}"

OS: macOS 11.2.3 / M1
Node: 15.12.0
NPM: 7.6.3
Rustc: 1.51.0

[lucasfernog]

Do you think you can make a PR fixing this? 🥺