fix who doesn't exist on some distros

If it doesn't exist, raise an exception.

lisa/tools/who.py

@@ -12,8 +12,9 @@ class Who(Tool):
     def command(self) -> str:
         return "who"
 
-    def _check_exists(self) -> bool:
-        return True
+    @property
+    def can_install(self) -> bool:
+        return False
 
     def last_boot(self, no_error_log: bool = True) -> datetime:
         command_result = self.run("-b", no_error_log=no_error_log)