Update GitHub Actions.

- Remove Ubuntu-16.04
- Add Ubuntu-20.04 {gcc,clang}

.github/workflows/build.yml

@@ -3,8 +3,8 @@ name: Build & Test
 on: [push, pull_request]
 
 jobs:
-  Ubuntu-1604:
-    runs-on: ubuntu-16.04
+  Ubuntu-1804-gcc:
+    runs-on: ubuntu-18.04
     env:
       MRUBY_CONFIG: ci/gcc-clang
       CC: gcc
@@ -17,8 +17,22 @@ jobs:
       - name: Build and test
         run: rake -m test:build && rake test:run
 
-  Ubuntu-1804-gcc:
+  Ubuntu-1804-clang:
     runs-on: ubuntu-18.04
+    env:
+      MRUBY_CONFIG: ci/gcc-clang
+      CC: clang
+    steps:
+      - uses: actions/checkout@v2
+      - name: Ruby version
+        run: ruby -v
+      - name: Compiler version
+        run: ${{ env.CC }} --version
+      - name: Build and test
+        run: rake -m test:build && rake test:run
+
+  Ubuntu-2004-gcc:
+    runs-on: ubuntu-20.04
     env:
       MRUBY_CONFIG: ci/gcc-clang
       CC: gcc
@@ -31,8 +45,8 @@ jobs:
       - name: Build and test
         run: rake -m test:build && rake test:run
 
-  Ubuntu-1804-clang:
-    runs-on: ubuntu-18.04
+  Ubuntu-2004-clang:
+    runs-on: ubuntu-20.04
     env:
       MRUBY_CONFIG: ci/gcc-clang
       CC: clang