ci: add nanvix-unstable compile checks to CI and Justfile (#1294)

Add CI coverage for the nanvix-unstable feature flag to catch
compilation regressions on the non-CoW code path.

Note: 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

Signed-off-by: danbugs <danilochiarlone@gmail.com>

Author: danbugs

Justfile

@@ -261,6 +261,7 @@ test-rust-tracing target=default-target features="":
 check-i686 target=default-target:
     cargo check -p hyperlight-common --target i686-unknown-linux-gnu --profile={{ if target == "debug" { "dev" } else { target } }}
     cargo check -p hyperlight-guest --target i686-unknown-linux-gnu --profile={{ if target == "debug" { "dev" } else { target } }}
+    cargo check -p hyperlight-common --target i686-unknown-linux-gnu --features nanvix-unstable --profile={{ if target == "debug" { "dev" } else { target } }}
     # Verify that trace_guest correctly fails on i686 (compile_error should trigger)
     ! cargo check -p hyperlight-guest --target i686-unknown-linux-gnu --features trace_guest --profile={{ if target == "debug" { "dev" } else { target } }} 2>/dev/null
 
@@ -283,6 +284,8 @@ check:
     {{ cargo-cmd }} check -p hyperlight-host --features print_debug  {{ target-triple-flag }}
     {{ cargo-cmd }} check -p hyperlight-host --features gdb  {{ target-triple-flag }}
     {{ cargo-cmd }} check -p hyperlight-host --features trace_guest,mem_profile  {{ target-triple-flag }}
+    {{ cargo-cmd }} check -p hyperlight-host --features nanvix-unstable  {{ target-triple-flag }}
+    {{ cargo-cmd }} check -p hyperlight-host --features nanvix-unstable,executable_heap  {{ target-triple-flag }}
 
 fmt-check:
     rustup +nightly component list | grep -q "rustfmt.*installed" || rustup component add rustfmt --toolchain nightly