The `no_std` and `no_main` attributes are only needed when we compile
the eBPF kernels for the "bpf" architecture. By feature-gating them on
the target architecture, we ensure the kernels compile just fine on a
host-architecture like x64. They won't be able to do anything
meaningfully but it is useful within the context of the larger workspace
they are embedded in as it allows `cargo build --workspace` and `cargo
test --workspace` to just work.
Replace all `assert!(matches!(..))` with `assert_matches!(..)`.
Remove the now-unused build-integration-test xtask command whose logic
doesn't match that of the build-and-run command.