fix(ci): add explicit rustup target add for cross-compilation

Ensures rust-std is installed for cross-compile targets (especially
x86_64-apple-darwin on arm64 macos-latest runners).

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: noahgift

.github/workflows/nightly.yml

@@ -48,6 +48,9 @@ jobs:
         with:
           targets: ${{ matrix.target }}
 
+      - name: Ensure target is installed
+        run: rustup target add ${{ matrix.target }}
+
       - name: Install cross
         if: matrix.use_cross
         run: cargo install cross --locked