Update Ubuntu versions and fix installation tests

Author: nobu

.github/workflows/ci.yml

@@ -27,8 +27,8 @@ jobs:
       matrix:
         os:
         - ubuntu-20.04
-        - ubuntu-18.04
-        - macos-10.15
+        - ubuntu-latest
+        - macos-latest
         - windows-latest
         ruby: ${{ fromJson(needs.ruby-versions.outputs.versions) }}
         include:
@@ -56,4 +56,4 @@ jobs:
       - run: rake build
 
       - run: gem install pkg/*.gem
-        if: ${{ ( matrix.ruby != 'debug' && matrix.os == 'linux-latest' ) || ( matrix.ruby != 'debug' && matrix.os == 'macos-latest' ) }}
+        if: ${{ matrix.ruby != 'debug' && ( matrix.os == 'ubuntu-latest' || matrix.os == 'macos-latest' ) }}