ci: test downstream sbpf-linker against workspace crates

[BretasArthur1]

Description

Adds a downstream workflow that builds and tests sbpf-linker at the sbpf-linker-next branch HEAD, against the sbpf crates in the checkout, so API breakage in the crates the linker consumes is caught in CI here, before a release is published.

How it works

1. Checks out sbpf-linker in sbpf-linker-next branch HEAD next to this repo.
2. Rewrites its sbpf-assembler/sbpf-common deps to use the checkout ones using cargo add --path.
3. Verifies the patch took effect, the job fails if either crate still resolves to the registry, so the silent fallback can never produce a false green.
4. Runs the linker's full test suite (compiletest + FileCheck over tests/assembly/*.rs), which exercises the assembler/common APIs e2e. Transitive workspace crates (sbpf-transform, sbpf-ir, sbpf-syscall-map) are verified along via the workspace's path deps.
5. A new comment triggered workflow was introduced for testing users fork with the compatible version of sbpf-linker against the current checkout. Usage: /test-sbpf-linker <owner>/<repo> <ref>

[BretasArthur1]

#159 broke compatibility with sbpf-linker when it moved opcode handling to tablegen. When sbpf releases a new version containing this new api, we should bump sbpf-linker deps and refactor the consumer in favor of the new api. This would fix it: blueshift-gg/sbpf-linker#50 and then we can have the downstream CI working.

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