workflows/tests: check tap_syntax on Linux too. (#54046)

We want to make sure all formulae are readable and pass style checks on
both macOS and Linux.

Why Linux? We're doing increasing numbers of things on Linux (bottle
uploads, formulae.brew.sh generation, etc.) and can no longer have
homebrew-core only work on macOS.

This means that changes that make formulae unreadable on Linux will need
to be fixed (e.g. https://github.com/Homebrew/brew/pull/7470) before the
formula changes are merged.

As a nice side-effect: this should make life easier for linuxbrew-core
maintainers.

.github/workflows/tests.yml

@@ -5,19 +5,45 @@ on:
   pull_request: []
 jobs:
   tap_syntax:
-    runs-on: macOS-latest
+    runs-on: ${{ matrix.os }}
+    strategy:
+      matrix:
+        os: [ubuntu-latest, macOS-latest]
     steps:
     - name: Set up Homebrew
       id: set-up-homebrew
       run: |
-        HOMEBREW_REPOSITORY="$(brew --repo)"
-        brew update-reset "$HOMEBREW_REPOSITORY"
+        if which brew &>/dev/null; then
+          HOMEBREW_REPOSITORY="$(brew --repo)"
+          brew update-reset "$HOMEBREW_REPOSITORY"
 
+          HOMEBREW_CORE_REPOSITORY="$HOMEBREW_REPOSITORY/Library/Taps/homebrew/homebrew-core"
+          ln -s "$PWD" "$HOMEBREW_CORE_REPOSITORY"
+        else
+          HOMEBREW_PREFIX=/home/linuxbrew/.linuxbrew
+          HOMEBREW_REPOSITORY="$HOMEBREW_PREFIX/Homebrew"
+          sudo mkdir -p "$HOMEBREW_PREFIX"
+          sudo git clone --depth=1 https://github.com/Homebrew/brew "$HOMEBREW_REPOSITORY"
+
+          HOMEBREW_CORE_REPOSITORY="$HOMEBREW_REPOSITORY/Library/Taps/homebrew/homebrew-core"
+          sudo mkdir -p "$HOMEBREW_CORE_REPOSITORY"
+          git -C "$HOMEBREW_CORE_REPOSITORY" init
+          git -C "$HOMEBREW_CORE_REPOSITORY" remote add origin "https://github.com/$GITHUB_REPOSITORY"
+
+          cd "$HOMEBREW_PREFIX"
+          sudo mkdir -p bin etc include lib opt sbin share var/homebrew/linked Cellar
+          sudo ln -sf ../Homebrew/bin/brew "$HOMEBREW_PREFIX/bin/"
+          cd -
+
+          export PATH="$HOMEBREW_PREFIX/bin:$PATH"
+          echo "::add-path::$HOMEBREW_PREFIX/bin"
+        fi
+
+        GEMS_PATH="$HOMEBREW_REPOSITORY/Library/Homebrew/vendor/bundle/"
+        echo "::set-output name=gems-path::$GEMS_PATH"
         GEMS_HASH=$(shasum -a 256 "$HOMEBREW_REPOSITORY/Library/Homebrew/Gemfile.lock" | cut -f1 -d' ')
         echo "::set-output name=gems-hash::$GEMS_HASH"
 
-        HOMEBREW_CORE_REPOSITORY="$(brew --repo homebrew/core)"
-
         cd "$HOMEBREW_CORE_REPOSITORY"
         rm -rf "$GITHUB_WORKSPACE"
         ln -s "$HOMEBREW_CORE_REPOSITORY" "$GITHUB_WORKSPACE"
@@ -25,24 +51,34 @@ jobs:
         git checkout --force -B master FETCH_HEAD
         cd -
 
-    - name: Install taps
-      run: |
-        brew tap homebrew/test-bot
-        brew update-reset Library/Taps/homebrew/homebrew-test-bot
+        if [ "$RUNNER_OS" = "Linux" ]; then
+          sudo chown -R "$USER" "$HOMEBREW_PREFIX"
+          sudo chmod -R g-w,o-w "$HOMEBREW_CORE_REPOSITORY"
+        fi
+
+    - name: Set up Ruby
+      if: matrix.os == 'ubuntu-latest'
+      uses: actions/setup-ruby@master
+      with:
+        ruby-version: '2.6'
 
     - name: Cache Bundler RubyGems
       id: cache
       uses: actions/cache@v1
       with:
-        path: /usr/local/Homebrew/Library/Homebrew/vendor/bundle/
+        path: ${{ steps.set-up-homebrew.outputs.gems-path }}
         key: ${{ runner.os }}-gems-${{ steps.set-up-homebrew.outputs.gems-hash }}
-        restore-keys: |
-          ${{ runner.os }}-gems-
+        restore-keys: ${{ runner.os }}-gems-
 
     - name: Install Bundler RubyGems
       if: steps.cache.outputs.cache-hit != 'true'
       run: brew install-bundler-gems
 
+    - name: Install taps
+      run: |
+        brew tap homebrew/test-bot
+        brew update-reset "$(brew --repo)/Library/Taps/homebrew/homebrew-test-bot"
+
     - name: Run brew test-bot --only-tap-syntax
       run: brew test-bot --only-tap-syntax