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 and storage path changes only; production nodes still fetch firecracker by name, so prod rollout behavior is unchanged.

Overview
Adds a second release build with the gdb feature that outputs firecracker-debug and firecracker-debug.debug, with the GNU debuglink pointed at the renamed DWARF file. The release workflow only skips per-arch builds when both firecracker-<arch> and firecracker-debug-<arch> are already on the release, uploads the debug release assets, and mirrors prod plus debug objects into GCS under separate names so prod clients still resolve only firecracker. validate.py and its tests treat missing debug assets as needing a build; upload-release-to-gcs.sh maps the new release asset names to the matching GCS paths.

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