Sunbeam doesn’t specify that this standard action provokes an attack of opportunity, and it doesn’t fall into any of the general categories of actions that provokes:
It isn’t an “attack (ranged),” since that is described as “With a ranged weapon, you can shoot or throw at any target […]” and the sunbeam beam isn’t a ranged weapon nor does it get shot or thrown (and beyond that, doesn’t involve a ranged attack roll).
It isn’t “cast a spell (1 standard action casting time),” since you already cast the spell earlier and are now just utilizing its effects.
Since sunbeam doesn’t say it provokes, and it isn’t any general category of action that provokes, it doesn’t provoke. This is the official rule, and it’s also the ruling you should generally make, because it’s an unfair “gotcha” to switch up the rules when it comes up in the middle of a fight—and sunbeam doesn’t get used very often, so in most cases it’s not something that a DM will have had an opportunity to establish a houserule about ahead of time. Generally speaking, maintaining the “predictability” of the rules is more important than any other consideration here, since whether or not it provokes is a minor thing.
That said, if one were establishing houserules about it upfront, it’s almost a ranged attack, and it’s kinda-sorta like casting a spell, so it might seem reasonable to rule that it should provoke like those things provoke. Counterarguments for that, narratively, are that the ranged attack requires careful aim, even up close, while this is a big blast of light you just have to point in the correct direction, and that you don’t have to perform any verbal or somatic components any longer, you just aim and fire. The more important counterargument, however, is for gameplay considerations: sunbeam is a weak, niche spell. Nerfing just makes it worse, so you shouldn’t do that. With a similar, but stronger, spell, maybe you would, but I wouldn’t recommend it with sunbeam.