ci: Fix ignored.yml and update FFI tests

[samuelburnham]

• Fixes the ignored-test CI job due to a lean-action bug, which will be fixed in fix: Set test-args input correctly and improve functional tests leanprover/lean-action#153.
• Changes the Valgrind job to only run the ffi test suite, as we are trusting Lean itself to be leak-free and only need to test our Rust FFI boundary. Running the full suite is also expensive at 30+ minutes per CI run due to Valgrind overhead.
• Adds more FFI tests for external Lean objects and Ixon environment serde
• Updates Blake3.lean to the latest commit, which fixed a memory leak

Successful run: https://github.com/argumentcomputer/ix/actions/runs/22956912799