try detect macOS in `initial_setup.sh`

/etc/os-release was invented by systemd. macOS doesn't have it.

Thankfully, the script equipped with code for situation where ID is undetected, so just set it to Unknown! is fine.

ID from /etc/os-release appears to be the only field used by this script.

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Background:

Currently, I have to spend lots of time with macOS and I want to test the feasibility of using KDE Builder on macOS.

It eventually works, here is kcalc on macOS 15 (Qt and dependencies obtained from homebrew, Python in venv):

And this is the first issue that I encountered.