Use PATH lookup for systemctl in systemd meter

Before this change, the systemd meter was broken on distros like NixOS,
which have systemctl in PATH, but not at /bin/systemctl. After the
change, it works on all my NixOS machines.

linux/SystemdMeter.c

@@ -219,8 +219,8 @@ static void updateViaExec(void) {
          exit(1);
       dup2(fdnull, STDERR_FILENO);
       close(fdnull);
-      execl("/bin/systemctl",
-            "/bin/systemctl",
+      execlp("systemctl",
+             "systemctl",
              "show",
              "--property=SystemState",
              "--property=NFailedUnits",