Add default test

.github/workflows/test_install_as_default_toolchain.yml

@@ -0,0 +1,57 @@
+name: CI Test - install as default toolchain
+on:
+  - pull_request
+jobs:
+  install_on_runner:
+    runs-on: ${{ matrix.os }}
+    strategy:
+      matrix:
+        os:
+          - ubuntu-latest
+          - macOS-latest
+          - windows-latest
+        toolchain:
+          - 1.70.0
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v4
+
+      - name: Use myself
+        id: toolchain
+        uses: ./
+        with:
+          toolchain: ${{ matrix.toolchain }}
+          profile: minimal
+          default: true
+
+      - name: Test toolchain version
+        run: |
+          test "${{ steps.toolchain.outputs.rustc }}" = "${{ matrix.toolchain }}" || (echo "Expected: ${{ matrix.toolchain }}; Got: ${{ steps.toolchain.outputs.rustc }}" && exit 1)
+
+  install_in_docker:
+    runs-on: ubuntu-latest
+    strategy:
+      matrix:
+        toolchain:
+          - 1.70.0
+    # Docker image, not the GitHub Actions VM
+    container: ubuntu:latest
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v4
+
+        # `rustup` will need `curl` or `wget` later
+      - name: Install packages
+        run: apt-get update && apt-get install -y curl
+
+      - name: Use myself
+        id: toolchain
+        uses: ./
+        with:
+          toolchain: ${{ matrix.toolchain }}
+          profile: minimal
+          default: true
+
+      - name: Test toolchain version
+        run: |
+          test "${{ steps.toolchain.outputs.rustc }}" = "${{ matrix.toolchain }}" || (echo "Expected: ${{ matrix.toolchain }}; Got: ${{ steps.toolchain.outputs.rustc }}" && exit 1)