Override host triple after rustup install

rustup-init bakes in x86_64-pc-windows-gnu on MSYS2 ARM.
Install first, then switch default host and toolchain to
aarch64-pc-windows-gnullvm.

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

Author: joske

.github/workflows/test-windows-arm.yml

@@ -42,8 +42,11 @@ jobs:
           export CARGO_HOME="$(pwd)/.cargo"
           export RUSTUP_HOME="$(pwd)/.rustup"
           rm -rf "$RUSTUP_HOME"
-          curl --proto '=https' --tlsv1.2 -sSf https://sh.rustup.rs | sh -s -- -y --default-host aarch64-pc-windows-gnullvm --default-toolchain stable
+          curl --proto '=https' --tlsv1.2 -sSf https://sh.rustup.rs | sh -s -- -y --default-toolchain stable
           export PATH="$(pwd)/.cargo/bin:$PATH"
+          rustup set default-host aarch64-pc-windows-gnullvm
+          rustup toolchain install stable-aarch64-pc-windows-gnullvm
+          rustup default stable-aarch64-pc-windows-gnullvm
       - name: Configure cargo linker
         shell: msys2 {0}
         run: |