Adapts to new Boost download location

[clonker]

Fixes #15690

PR switching to the new emscripten docker file: #15693

[github-actions]

solbuildpackpusher/solidity-buildpack-deps:emscripten-20 [solbuildpackpusher/solidity-buildpack-deps@sha256:98f963ed799a0d206ef8e7b5475f847e0dea53b7fdea9618bbc6106a62730bd2].

[ekpyron]

We also need a PR switching to the new docker file (and look at tests there to proceed with merging here)

We can keep ignoring the ossfuzz image for now, since IIRC it'll be separately fixed.

I'm not approving, since I don't see the sibling PR switching the image hashes for CI yet (if there is one, link it in the description)

[ekpyron]

Already approving this (even though it's technically not the correct sequence of changes), since the emscripten build in #15691 passed (we should wait for the tests with merging it, though).

Not sure if we should even touch the ossfuzz image here, but I guess better to avoid confusion about the URL here when finally fixing it properly, so fine with keeping the change here, even though we can't build the image.