Simplify os syntax in CI to avoid so many '-latest'

Author: eregon

.github/workflows/test.yml

@@ -14,16 +14,16 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        os: [ ubuntu-latest, macos-latest, windows-latest ]
+        os: [ ubuntu, macos, windows ]
         # Use various version syntaxes here for testing
         ruby: [ 2.2, 2.3, 2.4, 2.5, 2.6.6, 2.7, ruby-head, jruby-9.1, jruby, jruby-head, truffleruby, truffleruby-head ]
         exclude:
-        - os: windows-latest
+        - os: windows
           ruby: truffleruby
-        - os: windows-latest
+        - os: windows
           ruby: truffleruby-head
     name: ${{ matrix.os }} ${{ matrix.ruby }}
-    runs-on: ${{ matrix.os }}
+    runs-on: ${{ matrix.os }}-latest
     steps:
     - uses: actions/checkout@v2
     - uses: ./
@@ -76,11 +76,11 @@ jobs:
       fail-fast: false
       matrix:
         cfg:
-        - { ruby: rubinius, os: ubuntu-18.04   }
-        - { ruby: mingw   , os: windows-latest }
-        - { ruby: mswin   , os: windows-latest }
+        - { ruby: rubinius, os: ubuntu  }
+        - { ruby: mingw   , os: windows }
+        - { ruby: mswin   , os: windows }
     name: ${{ matrix.cfg.os }} ${{ matrix.cfg.ruby }}
-    runs-on: ${{ matrix.cfg.os }}
+    runs-on: ${{ matrix.cfg.os }}-latest
     steps:
     - uses: actions/checkout@v2
     - uses: ./