fix(ci): restore Rust target triple and separate cross-compilation setup

Blacksmith migration (#2414) incorrectly rewrote the Rust target triple
aarch64-unknown-linux-gnu to the runner label blacksmith-4vcpu-ubuntu-2404-arm.
Restore the correct Rust target and split cross-compilation target addition
into an explicit rustup command.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

.github/workflows/build-native.yml

@@ -31,7 +31,7 @@ jobs:
             target: x86_64-unknown-linux-gnu
             platform: linux-x64-gnu
           - os: ubuntu-latest
-            target: blacksmith-4vcpu-ubuntu-2404-arm
+            target: aarch64-unknown-linux-gnu
             platform: linux-arm64-gnu
             cross: true
           - os: windows-latest