ci: Add ubuntu-20.04 to matrix (#336)

https://github.com/actions/virtual-environments/issues/228#issuecomment-637362224

.github/workflows/test.yml

@@ -22,6 +22,7 @@ jobs:
     strategy:
       matrix:
         os:
+          - 'ubuntu-20.04'
           - 'ubuntu-18.04'
           - 'ubuntu-16.04'
           - 'macos-latest'
@@ -168,71 +169,10 @@ jobs:
           user_email: 'github-actions[bot]@users.noreply.github.com'
           # commit_message: ${{ github.event.head_commit.message }}
 
-  test-ubuntu-2004:
-    runs-on: 'ubuntu-18.04'
-    if: contains(github.event.head_commit.message, '[skip ci]') == false
-    container:
-      image: 'ubuntu:20.04'
-    steps:
-      - name: Install deps
-        run: |
-          apt update
-          apt install -y software-properties-common
-          add-apt-repository ppa:git-core/ppa
-          apt update
-          apt install -y \
-            git \
-            ssh
-
-      - uses: actions/checkout@v2
-
-      - name: Dump GitHub context
-        env:
-          GITHUB_CONTEXT: ${{ toJson(github) }}
-        run: echo "${GITHUB_CONTEXT}"
-
-      - name: Read .nvmrc
-        run: echo "::set-output name=NVMRC::$(cat .nvmrc)"
-        id: nvm
-
-      - name: Setup Node
-        uses: actions/setup-node@v2.0.0
-        with:
-          node-version: '${{ steps.nvm.outputs.NVMRC }}'
-
-      - run: npm i -g npm
-
-      - name: Dump version
-        run: |
-          node -v
-          npm --version
-          git --version
-
-      - run: npm ci
-      - run: npm test
-
-      - name: Run ncc
-        run: npm run build
-
-      - name: Remove lint-staged husky
-        if: github.ref == 'refs/heads/master'
-        run: |
-          npm uninstall lint-staged husky
-          git checkout package-lock.json package.json
-
-      - name: Setup mdBook
-        if: github.ref == 'refs/heads/master'
-        uses: peaceiris/actions-mdbook@v1
-        with:
-          mdbook-version: '0.3.7'
-
-      - name: Build site
-        if: github.ref == 'refs/heads/master'
-        working-directory: ./test_projects/mdbook
-        run: mdbook build
-
       - name: Deploy
-        if: github.ref == 'refs/heads/master'
+        if: |
+          startsWith(matrix.os, 'ubuntu-20.04') &&
+          github.ref == 'refs/heads/master'
         uses: ./
         with:
           # deploy_key: ${{ secrets.ACTIONS_DEPLOY_KEY }}