It has never been explicitly said who slowed down the self destruct countdown, but for sure it wasn't Daniel Jackson. I'll be quoting some parts of this transcript of the episode.
Post climax scene in the briefing room, where Daniel himself says it wasn't him:
CARTER: I still don't quite understand what happened. One minute Anubis is about to push the button that ends all life in the galaxy, and the next minute he's just ... gone.
TEAL'C: Indeed. It is a great mystery.
BRA'TAC: One can only assume he was vanquished by some beings. If not, why would he forfeit the weapon and his army?
(Sam looks at Jack.)
CARTER: You think?
O'NEILL: I do.
CARTER: It's the only thing that would explain the self destruct not going off.
BRA'TAC: Of what do you speak?
TEAL'C: O'Neill believes that Daniel Jackson was somehow responsible.
JACKSON (offscreen): No! It wasn't me.
As to who stopped/slowed down the clock, we can only assume it was Oma (at least that's my take on it):
ANUBIS: You can't kill me either.
OMA: I can fight you.
ANUBIS: Well, you can't win.
OMA: It won't matter. You won't be able to do anything but fight me back.
ANUBIS: Well, what are you gonna do?
OMA: Something I should have done a long time ago. (She starts to walk towards him.)
ANUBIS: Oh, no. No. No! No!
(He raises his hands to try to stop her. She morphs into white light. As she reaches him, he too morphs into white light. They merge, and the light rises up and goes through the ceiling of the diner. Daniel and all the customers stare up at the ceiling. Daniel walks to the spot that they disappeared from and looks around at the customers, bewildered.)
S.G.C. CONTROL ROOM. The self destruct has reached 7 seconds. Everyone is staring around anxiously. As the self destruct reaches 1:30, the countdown starts to slow down. It takes an age to tick from 1:27 to 1:26. A couple of seconds later, the Gate shuts down.
HARRIMAN: Wormhole disengaged.
CARTER: What's going on?
HARRIMAN: I don't know. Must be some kind of ... system malfunction.
CARTER: That's impossible!
O'NEILL: Shut it off.
(Sam reaches down and types.)
CARTER: Aborting self destruct. (She finishes typing and the self destruct shuts down. She looks at Jack, who blows out a relieved breath.)
It can't be a coincidence that the clock malfunctioned at the same time Oma was fighting Anubis. Also he was only a half-ascended being, unlike Oma the full-blown Ancient, so she probably would have the power to fight Anubis and stop the self destruct. Maybe she even used the fight to hide the fact of helping less evolved beings from the other Ancients, and we all now how they feel about it...