fix mac binary not placed in a top level directory

[umbynos]

This PR fixes the mac binary not being placed in a top level directory as the other binaries.
Before:

$ tar -tvf imgtool_1.8.0-arduino.1_macOS_64bit.tar.gz
-rwxr-xr-x runner/staff 8570880 2022-05-31 12:03 imgtool
-rw-r--r-- runner/staff   11527 2022-05-31 12:01 LICENSE.txt

After:

$ tar -tvf imgtool_1.8.0-arduino.2-rc2_macOS_64bit.tar.gz
drwxr-xr-x runner/staff      0 2022-07-18 11:04 imgtool_macOS_64bit/
-rwxr-xr-x runner/staff 8576528 2022-07-18 11:04 imgtool_macOS_64bit/imgtool
-rw-r--r-- runner/staff   11527 2022-07-18 11:04 imgtool_macOS_64bit/LICENSE.txt

[per1234]

Thanks Umberto!

I like that the tar command is more simple now. I always get confused about the function of the -C flag in these repackaging steps.