ci: fix macOS rustup cache corruption in install-rust action

Switch back to `rustup toolchain install` (the rustup-documented way to
install the toolchain pinned by rust-toolchain.toml across rustup 1.28+
versions) and disable `cache-bin` on macOS.

Caching ~/.cargo/bin caused macOS jobs to fail with
`error: unexpected argument 'fmt' found / Usage: rustup-init[EXE]`:
rust-cache was restoring a stale cargo proxy from a runner with a
different rustup version, overwriting the proxy installed in this run
and leaving cargo behaving like rustup-init.

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

Author: GuillaumeLagrange

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

@@ -14,11 +14,13 @@ runs:
   steps:
     - name: Install rust-toolchain.toml
       shell: bash
-      run: rustup show
+      run: rustup toolchain install
     - name: Install additional rustup components
       if: inputs.components != ''
       shell: bash
       run: echo "${{ inputs.components }}" | tr ',' '\n' | xargs rustup component add
     - uses: Swatinem/rust-cache@c19371144df3bb44fab255c43d04cbc2ab54d1c4 # v2
       with:
         key: ${{ inputs.cache-key }}
+        # https://github.com/Swatinem/rust-cache/issues/341
+        cache-bin: ${{ runner.os != 'macOS' }}