Check for fexistence before checking if executable

See #2756

src/core/Kernel.pm6

@@ -11,7 +11,7 @@ class Kernel does Systemic {
     has Bool $!has_uname;
 
     method !uname($opt) {
-        $!has_uname //= "/bin/uname".IO.x || "/usr/bin/uname".IO.x;
+        $!has_uname //= ("/bin/uname", "/usr/bin/uname").first({ .IO.e && .IO.x });
         $!has_uname ?? qqx/uname $opt/.chomp !! 'unknown';
     }