Create inner workspace for rust guests to reduce compile time

[ludfjig]

Reduces time to build rust guests by putting them in a workspace together. This avoids rebuilding hyperlight-libc from scratch and hyperlight-guest-bin from scratch for each guest which is very slow. On my machine just guests time is reduced by 6 minutes.

Aside: rust guests shouldn't need hyperlight-libc (#1483), but this is a good change regardless since hyperlight-guest-bin will be shared even if the libc issue is fixed in the future

[Copilot]

Pull request overview

Creates a nested Cargo workspace under src/tests/rust_guests/ so the Rust guest crates build together and can share build artifacts (notably hyperlight-libc / hyperlight-guest-bin), reducing repeated recompilation and improving just guests build time.

Changes:

• Introduce an inner Cargo workspace for Rust guest crates and consolidate to a single src/tests/rust_guests/Cargo.lock.
• Update Justfile, CI workflows, Nix flake inputs, and Dependabot configuration to build/fetch/cache against the guest workspace rather than per-guest crates.
• Remove per-guest Cargo.lock files now that dependency resolution is centralized at the guest workspace root.

Reviewed changes

Copilot reviewed 11 out of 14 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
src/tests/rust_guests/simpleguest/Cargo.lock	Removed per-crate lockfile in favor of workspace lock.
src/tests/rust_guests/dummyguest/Cargo.lock	Removed per-crate lockfile in favor of workspace lock.
src/tests/rust_guests/Cargo.toml	New nested workspace definition for Rust guest crates.
src/tests/rust_guests/Cargo.lock	Updated lockfile to represent the combined guest workspace resolution.
Justfile	Build/clean/fetch/fmt/clippy paths updated to operate on the guest workspace and shared target dir.
flake.nix	Switch Nix vendoring/importCargoLock inputs from per-guest manifests to the guest workspace manifest.
dev/check-license-headers.sh	Adjust target-dir exclusion to match the new shared guest workspace target/ location.
Cargo.toml	Exclude the nested src/tests/rust_guests workspace from the root workspace.
.github/workflows/ValidatePullRequest.yml	Update comments/intent around updating the guest workspace lock on Dependabot PRs.
.github/workflows/RustNightly.yml	Update caches to the unified guest workspace target paths.
.github/workflows/dep_update_guest_locks.yml	Update workflow to refresh/commit only the guest workspace Cargo.lock.
.github/workflows/dep_code_checks.yml	Point guest lock fetch and caching to the guest workspace.
.github/workflows/dep_build_guests.yml	Point guest build caching to the guest workspace.
.github/dependabot.yml	Consolidate guest Dependabot config to a single /src/tests/rust_guests workspace entry.

[simongdavies]

Nice!! LGTM