pre-built kernel with FUSE enabled for integration testing

For now, we've added building a kernel with FUSE enabled as a step ran in every integration test on Buildkite (#659). This adds about 1-2 minutes to the overall build time. Ideally, we would like to have a pre-built kernel that we download for integration testing, like the previous behavior with default-vmlinux: https://github.com/firecracker-microvm/firecracker-containerd/blob/3f792eb2c14ed3f057963b4ed74d7601974f6304/Makefile#L211-L216

Start with the Makefile around lines 211-216 and the Buildkite integration-test step introduced in #659. Trace how default-vmlinux was previously downloaded, then make the integration tests reuse a pre-built FUSE-enabled kernel; done means the kernel is fetched rather than rebuilt for every test and the existing integration tests pass.

Written by the indexing model from the issue text.