feat: publish a debug-only firecracker binary (gdb-enabled)

[kalyazin]

For each FC version, also build and publish a debug variant alongside the prod binary: a release (optimized) build with debug symbols kept and the upstream gdb feature enabled (--features gdb), plus its split DWARF companion. Used only for debugging guest kernels on dev nodes.

Published as firecracker-debug (+ firecracker-debug.debug) next to firecracker. Client nodes resolve the FC binary at exactly <version>/<arch>/firecracker, so the differently-named debug artifacts are never pulled by prod; the dev debugging workflow fetches them explicitly, matched to a snapshot's firecracker version.

• build.sh: second build with --features gdb, emit firecracker-debug + repoint the debuglink to the renamed DWARF companion.
• release.yml: build skip-check requires both prod and debug assets; publish attaches firecracker-debug-[.debug]; deploy uploads them to each env's GCS bucket at the firecracker-debug[.debug] name.
• upload-release-to-gcs.sh: also uploads the debug artifacts to the parallel path.

[cursor]

PR Summary

Low Risk
CI/build and storage path changes only; prod clients still resolve firecracker and are not pointed at the debug binaries.

Overview
Adds a second optimized Firecracker build with --features gdb, emitting firecracker-debug and split DWARF, and wires release/GCS upload for those names alongside prod firecracker.

check_artifacts_needed and the workflow build skip step only require firecracker-<arch> and firecracker-debug-<arch>, not firecracker-debug-<arch>.debug, so a release can be considered complete and skip rebuilding when the DWARF companion is still missing on the release.

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[cursor]

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