chore(rust): ignore Cargo target artifacts

Keep local Rust build outputs out of the migration diff. The static archive is
rebuilt by the validation workflow and must not be tracked as source.

Test Plan:
- git check-ignore -v rust/target

Refs: rust/README.md

.gitignore

@@ -33,6 +33,7 @@ cmakebuild/
 install/
 
 # Build artefacts
+/rust/target/
 contrib/linux-kernel/linux/
 projects/
 bin/