Updating asset replacer body logic

[leanderiversen]

Implementing the asset replacer changes suggested by @brentrager in #32.

Closes #42.

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[jayair]

So how does this fix your issue btw?

[leanderiversen]

I'm unable to explain the mechanics of the fix, as it was first presented by @brentrager in #32, but it seems like it's relating to timing out somehow. Our deployments can last anywhere from 10 to 25 minutes depending on the environment, but not sure if this is the actual issue, as the Next.js build is what takes the longest.

We have been running the patched version for 3 days at the time of writing, with about 50 deployments and no failures. Others in #32 have also been running their own patched versions successfully, but I cannot explain exactly why it works.

It was initially thought that #34 on its own would fix the issue, but it seems like a combination of the two is required for successful deployments.