ci: add nanvix-unstable compile checks to CI and Justfile

[danbugs]

Summary

Adds compile-time CI coverage for the nanvix-unstable feature flag introduced in #1291 to catch regressions on the non-CoW code path.

Only compile checks are added — not test runs — because the standard Hyperlight test guests require CoW paging, which is disabled under nanvix-unstable.

Changes

• Justfile check: verify nanvix-unstable and nanvix-unstable,executable_heap compile on the host
• Justfile check-i686: verify hyperlight-common builds for i686 with nanvix-unstable enabled

The check recipe is already invoked by both test-like-ci and code-checks-like-ci, so these checks run automatically in CI without needing a new workflow step.

[jsturtevant]

should we update clippy-exhaustive as well?

[danbugs]

should we update clippy-exhaustive as well?

clippy-exhaustive already tests w/ nanvix-unstable. clippy-exhaustive auto-discovers features to test w/ using cargo metadata. Verified it does pick it up locally rn:

❯ cargo metadata --format-version 1 --no-deps | jq -r '.packages[] | select(.name == "hyperlight-host") | .features | keys[]'
build-metadata
crashdump
default
executable_heap
function_call_metrics
fuzzing
gdb
kvm
mem_profile
mshv3
nanvix-unstable
print_debug
trace_guest