ci: check lockfile of difftests

Author: Firestar99

.github/workflows/ci.yaml

@@ -185,7 +185,9 @@ jobs:
         run: cargo version
       - name: cargo fetch --locked
         run: cargo fetch --locked --target ${{ matrix.target }}
-      - name: difftest
+      - name: cargo fetch --locked difftests
+        run: cargo fetch --locked --manifest-path=tests/difftests/tests/Cargo.toml --target ${{ matrix.target }}
+      - name: difftests
         run: cargo run -p difftests --release --no-default-features --features "use-installed-tools"
 
   # This allows us to have a single job we can branch protect on, rather than needing