Use uname -s to detect system name

GNU style uname supports -o option, but AIX system's does not. Use -s to
get system name, which is supported by both.

os_info/src/aix/mod.rs

@@ -29,7 +29,7 @@ fn get_version() -> Option<String> {
 }
 
 fn get_os() -> Type {
-    match uname("-o").as_deref() {
+    match uname("-s").as_deref() {
         Some("AIX") => Type::AIX,
         _ => Type::Unknown,
     }