Revert "Swap the mirroring order."

[godofredoc]

Reverts #4650

[flutter-dashboard]

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[stuartmorgan-g]

I looked at the implementation of the mirror script a bit, and it has:

            result = await octokit.git.updateRef({
                ...github.context.repo,
                ref: `heads/${destBranchName}`,
                sha: sourceBranchSha,
                force: true,
            });

I haven't looked at the internals of that, but the force: true makes me think we don't want to do this. I'm concerned that this will destroy main.

[stuartmorgan-g]

If it is indeed doing a force-push to the target branch, then all we need to do is land #4654 and it'll stomp master.

[stuartmorgan-g]

If it is indeed doing a force-push to the target branch, then all we need to do is land #4654 and it'll stomp master.

This was the case, so this PR isn't needed.