fix: kill zombie builds if they complete

[deer]

I was working on a dependency visualizer in order to learn more about deno_graph and various network diagram visualization libraries. I've started out with vis-network, but I quickly encountered a problem when deploying: despite the build() function in build.ts finishing, the process doesn't end!

This is different than #2240 because... I dunno, I just think it is. I've gated the code in the fixture with the standard advice: !Deno.args.includes("build"), but this does not help. But if you comment out Deno.exit() then it starts hanging again. And going a step further, commenting out the code in the effect like this:

      // network.current = new Network(container.current, {
      //   nodes: nodes,
      //   edges: edges,
      // }, {});

causes the build to finish, even though that code isn't even running on the server.

Perhaps I'm going about this wrong and I should instead be importing vis-network dynamically, if we're not in build mode? I'm really unsure what's going on here, but using the code in the forked branch unblocked my project, so I thought I would clean it up enough and write a test -- perhaps it's useful for someone else.

[deer]

Well, this fixes my problem and passes on my computer. I'm not sure what's going on here. I'll look at the failures in more detail tomorrow.