feat(install-toolchain): support Windows, incl. windows-11-arm

windows-11-arm ships no Rust (actions/partner-runner-images#77), and the Unix sh.rustup.rs installer doesn't apply there.
When no rustup is present on Windows, fetch the arch-appropriate rustup-init.exe instead - reading the true machine arch from PROCESSOR_ARCHITE(W6432), since git-bash's `uname -m` lies under x64 emulation.
Add a windows-latest + windows-11-arm self-test leg.

Author: gronke

.github/actions/install-toolchain/action.yml

@@ -1,8 +1,9 @@
 name: Install Rust toolchain
 description: >-
   Install Rust via rustup (toolchain, components, targets) and put
-  ~/.cargo/bin on PATH. Avoids third-party toolchain actions so behaviour does
-  not drift with the runner image.
+  ~/.cargo/bin on PATH. Works on Linux, macOS and Windows — including
+  windows-11-arm, which ships no preinstalled Rust. Avoids third-party
+  toolchain actions so behaviour does not drift with the runner image.
 
 inputs:
   toolchain:

.github/actions/install-toolchain/install-toolchain.sh

@@ -9,10 +9,32 @@ if command -v rustup >/dev/null 2>&1; then
   rustup toolchain install "$TOOLCHAIN" --profile minimal
   rustup default "$TOOLCHAIN"
 else
-  curl --proto '=https' --tlsv1.2 -sSf https://sh.rustup.rs \
-    | sh -s -- -y --default-toolchain "$TOOLCHAIN" --profile minimal --no-modify-path
-  echo "$HOME/.cargo/bin" >> "$GITHUB_PATH"
-  export PATH="$HOME/.cargo/bin:$PATH"
+  case "$(uname -s)" in
+    MINGW* | MSYS* | CYGWIN*)
+      # Windows runners — notably windows-11-arm — can ship no Rust at all, and the Unix
+      # sh.rustup.rs installer doesn't apply here. Fetch the arch-appropriate rustup-init.exe.
+      # git-bash's `uname -m` can't be trusted under x64 emulation, so read the true machine arch
+      # from Windows' own PROCESSOR_ARCHITE(W6432) variables.
+      winarch="${PROCESSOR_ARCHITEW6432:-${PROCESSOR_ARCHITECTURE:-$(uname -m)}}"
+      case "$winarch" in
+        ARM64 | arm64 | aarch64) host=aarch64-pc-windows-msvc ;;
+        *) host=x86_64-pc-windows-msvc ;;
+      esac
+      curl --proto '=https' --tlsv1.2 -sSfL \
+        "https://static.rust-lang.org/rustup/dist/$host/rustup-init.exe" -o rustup-init.exe
+      ./rustup-init.exe -y --default-toolchain "$TOOLCHAIN" --profile minimal --no-modify-path
+      # GITHUB_PATH wants a Windows path (later steps may run in pwsh); this script continues in
+      # git-bash, where the cargo bin dir is $HOME/.cargo/bin.
+      printf '%s\\.cargo\\bin\n' "${USERPROFILE:-$HOME}" >> "$GITHUB_PATH"
+      export PATH="$HOME/.cargo/bin:$PATH"
+      ;;
+    *)
+      curl --proto '=https' --tlsv1.2 -sSf https://sh.rustup.rs \
+        | sh -s -- -y --default-toolchain "$TOOLCHAIN" --profile minimal --no-modify-path
+      echo "$HOME/.cargo/bin" >> "$GITHUB_PATH"
+      export PATH="$HOME/.cargo/bin:$PATH"
+      ;;
+  esac
 fi
 
 if [ -n "$COMPONENTS" ]; then

.github/workflows/selftest.yml

@@ -113,6 +113,30 @@ jobs:
           working-directory: fixtures/sample-crate
           expected-version: "0.1.0"
 
+  # install-toolchain must also work on Windows. windows-latest exercises the rustup-present path;
+  # windows-11-arm ships no Rust at all, exercising the rustup-init.exe bootstrap. Building the
+  # fixture proves the toolchain is usable, not merely installed (and, on arm64, that the image
+  # provides the MSVC arm64 link tools).
+  selftest-windows:
+    name: install-toolchain · ${{ matrix.runner }}
+    strategy:
+      fail-fast: false
+      matrix:
+        runner: [windows-latest, windows-11-arm]
+    runs-on: ${{ matrix.runner }}
+    steps:
+      - uses: actions/checkout@v6
+      - uses: ./.github/actions/install-toolchain
+        with:
+          components: clippy
+      - name: toolchain is installed + on PATH
+        shell: bash
+        run: rustc --version && cargo --version && cargo clippy --version
+      - name: build the fixture
+        shell: bash
+        working-directory: fixtures/sample-crate
+        run: cargo build --locked
+
   # rust-cache must OBEY an explicit CARGO_INCREMENTAL (not force it off): set it at job level and
   # confirm the action leaves it untouched for the build steps that would follow.
   selftest-cache-passthrough:

README.md

@@ -12,6 +12,7 @@ During bring-up, pin `@main`; once the interface is stable we cut a `v1` tag plu
 ### `install-toolchain`
 
 Installs Rust via rustup — toolchain, components, and targets — and puts `~/.cargo/bin` on `PATH`.
+Works on Linux, macOS and Windows; when a runner ships no Rust at all (e.g. `windows-11-arm`) it bootstraps the arch-appropriate `rustup-init` instead of failing.
 
 ```yaml
 - uses: gronke/cicd-rust/.github/actions/install-toolchain@main