Immediately flush output to stdout/stderr so that logs are not mixed on CI

Set in build configuration to enable on all CI platforms.

.github/workflows/build.yml

@@ -87,7 +87,7 @@ jobs:
       - name: Build and test
         # If build and test are separated like `rake && rake test`, somehow
         # it will be fully built even at `rake test`, so it is not separated.
-        run: rake -E STDOUT.sync=true test
+        run: rake test
 
   Windows-Cygwin:
     needs: Check-Skip
@@ -130,9 +130,7 @@ jobs:
         run: ${{ env.CC }} --version
       - name: Build and test
         shell: cmd
-        run: |
-          ruby /usr/bin/rake -E STDOUT.sync=true -m
-          ruby /usr/bin/rake -E STDOUT.sync=true test
+        run: ruby /usr/bin/rake -m && ruby /usr/bin/rake test
       - name: Set PATH for cache archiving (tar)
         # set Windows path so that Cygwin tar is not used for cache archiving
         run: echo '::set-env name=PATH::C:\windows\System32'
@@ -150,4 +148,4 @@ jobs:
         shell: cmd
         run: |
           call "C:\Program Files (x86)\Microsoft Visual Studio\2019\Enterprise\VC\Auxiliary\Build\vcvars64.bat"
-          rake -E STDOUT.sync=true -m && rake -E STDOUT.sync=true test
+          rake -m && rake test