"--attempt-instant-ddl" flag causing some issues with hooks and is incompatible with --execute flag

[YazanNoon]

when I tried to use the --attempt-instant-ddl flag, I faced two issues:

• the on-success hook will not trigger after the migration is done.
• even if I don't use the --execute flag, the migration will still happen anyway.

[hakusaro]

If it's not too much effort @timvaillancourt, I would suggest adding this to the docs around --attempt-instant-ddl, as looking at this, the original PR, and other issues led me to pulling this flag out of our production gh-ost scripts for safety.

I know you're very busy, and this is a low priority task, however.

[luddd3]

I just also got bitten by the fact that the migration is executed without the "--execute" flag when "--attempt-instant-ddl" is used.

[timvaillancourt]

@YazanNoon thanks for the report. The 2nd issue sounds dangerous so I have made a fix for that first here. If you have time to help validate the fix before/after merge, that would be very helpful 🙇

cc @hakusaro / @luddd3

[YazanNoon]

thanks a lot @timvaillancourt!
the PR looks good!

[hakusaro]

Thanks so much! It looks good to me! ^_^

[luddd3]

Thank you!