Re-include c89 job in CI.

[duesee]

As discussed with @franziskuskiefer, I'll remove the c89 job from CI for now. This job can be re-included when the issues blocking a successful build are resolved.

# XXX: This fails because c89 is utterly broken. We might want to do this at
  #      some point.
  c89:
    runs-on: ubuntu-18.04
    env:
      CC: gcc-4.8
      CXX: g++-4.8
    steps:
      - name: Output name
        id: vars
        run: |
          echo ::set-output name=short_sha::${GITHUB_SHA: -8}
      - name: Setup
        run: |
          sudo apt-get update
          sudo apt-get install ninja-build gcc-4.8 g++-4.8
      - uses: actions/checkout@v2
      - name: Debug Build
        run: ./mach build -v ${{ matrix.options }} -e c89 || true
      # - name: Release Build
      #   run: ./mach build --release -v ${{ matrix.options }} -e c89
      # - name: Install
      #   run: |
      #     mkdir pkg-release
      #     ./mach install -p $PWD/pkg-release -c Release
      #     mkdir pkg-debug
      #     ./mach install -p $PWD/pkg-debug
      # - name: Upload Artifacts
      #   uses: actions/upload-artifact@v2
      #   with:
      #     name: linux-x64-gcc48-c89-${{ steps.vars.outputs.short_sha }}${{ matrix.options }}
      #     path: |
      #       pkg-debug
      #       pkg-release
      #       build/Debug
      #       build/Release