std.zig.system.darwin.isSdkInstalled: fix implementation

* don't assert that the child process doesn't crash
* don't give a false negative on warnings printed to stderr

Also fix getSdk from the same file in the same way
c012f5d55dAndrew Kelley committed on 8/3/2023, 4:52:15 PMparente582a36
1 file changedLine totals unavailable