Fix Windows arch detection in install_cargo_binstall

archive_for_platform derived the Windows archive name from `uname -m`,
but Git for Windows' MSYS runtime does not report the host's real
architecture there: it reports the emulated x86_64 process architecture
under WOW64, or "unknown" for a native arm64 host with no case in its
uname implementation. On windows-11-arm this asked for
cargo-binstall-unknown-pc-windows-msvc.zip, which has no pinned digest,
and the script exited before installing anything.

Derive the Windows architecture from PROCESSOR_ARCHITEW6432 (set only
under WOW64/x64 emulation, to the true native architecture) falling
back to PROCESSOR_ARCHITECTURE instead, so both a native arm64 host and
an x64-emulated bash on an arm64 host resolve to the aarch64 archive.
The x64 leg still resolves to the same x86_64 archive as before.

Author: warp-agent-staging[bot]

script/install_cargo_binstall

@@ -43,7 +43,17 @@ archive_for_platform() {
             echo "cargo-binstall-$(uname -m)-unknown-linux-musl.tgz" ;;
         *)
             if [ "${OS-}" = "Windows_NT" ]; then
-                echo "cargo-binstall-$(uname -m)-pc-windows-msvc.zip"
+                # uname -m is unreliable here: Git for Windows' MSYS runtime reports the
+                # emulated x86_64 host under WOW64, or "unknown" on a native arm64 host,
+                # rather than the real machine architecture. PROCESSOR_ARCHITEW6432 (set
+                # only under emulation, to the true native architecture) falling back to
+                # PROCESSOR_ARCHITECTURE reports it correctly in both cases.
+                case "${PROCESSOR_ARCHITEW6432:-${PROCESSOR_ARCHITECTURE-}}" in
+                    ARM64)
+                        echo "cargo-binstall-aarch64-pc-windows-msvc.zip" ;;
+                    AMD64)
+                        echo "cargo-binstall-x86_64-pc-windows-msvc.zip" ;;
+                esac
             fi
             ;;
     esac