Add test manually downloading the built ruby

Author: eregon

.github/workflows/direct.yml

@@ -0,0 +1,19 @@
+name: Test manual setup
+on: [push]
+jobs:
+  test:
+    strategy:
+      fail-fast: false
+      matrix:
+        os: [ 'ubuntu-16.04', 'ubuntu-18.04', 'macos-latest' ]
+        ruby: [ 'ruby-2.6.5', 'ruby-2.7.0' ]
+    runs-on: ${{ matrix.os }}
+    steps:
+    - run: echo ::set-env name=HOME::$HOME
+    - run: mkdir ~/.rubies
+    - run: wget -O ~/.rubies/${{ matrix.ruby }}-${{ matrix.os }}.tar.gz https://github.com/eregon/ruby-install-builder/releases/download/builds/${{ matrix.ruby }}-${{ matrix.os }}.tar.gz
+    - run: tar xf ${{ matrix.ruby }}-${{ matrix.os }}.tar.gz
+      working-directory: ${{ env.HOME }}/.rubies
+    - run: echo "::add-path::${{ env.HOME }}/.rubies/${{ matrix.ruby }}/bin"
+    - run: ruby -v
+    - run: ruby -ropen-uri -e 'puts open("https://rubygems.org/") { |f| f.read(2014) }'