condition: when reading /etc/ modification timestamp, let's actualy compare it as-is

Previously, we'd only compare the nsec component of it, which sounds
needlessly fragile. Let's instead compare the timestamp as it is.

src/shared/condition.c

@@ -614,7 +614,7 @@ static int condition_test_needs_update(Condition *c, char **env) {
                         return true;
                 }
 
-                timespec_store(&other.st_mtim, timestamp);
+                return timespec_load_nsec(&usr.st_mtim) > timestamp;
         }
 
         return usr.st_mtim.tv_nsec > other.st_mtim.tv_nsec;