Install rustup if not available, Linux only

The original code stems from this PR:
https://github.com/dtolnay/rust-toolchain/pull/8

action.yml

@@ -47,6 +47,14 @@ runs:
         targets: ${{inputs.target}}
         components: ${{inputs.components}}
       shell: bash
+    - name: Install rustup, if needed
+      run: |
+        if ! command -v rustup &> /dev/null ; then
+          curl --proto '=https' --tlsv1.2 --retry 10 --retry-connrefused -fsSL "https://sh.rustup.rs" | sh -s -- --default-toolchain none -y
+          echo "${CARGO_HOME:-~/.cargo}/bin" >> $GITHUB_PATH
+        fi
+      if: runner.os != 'Windows'
+      shell: bash
     - name: rustup toolchain install ${{inputs.toolchain}}
       run: |
         rustup toolchain install ${{inputs.toolchain}}${{steps.flags.outputs.targets}}${{steps.flags.outputs.components}} --profile minimal${{steps.flags.outputs.downgrade}} --no-self-update