Install `bison` on the platforms.

.github/workflows/build.yml

@@ -90,8 +90,10 @@ jobs:
         run: ruby -v
       - name: Compiler version
         run: ${{ env.CC }} --version
-      - name: Install Bison
-        run: pacman -S bison
+      - uses: msys2/setup-msys2@v2
+        with:
+          update: true
+          install: 'git base-devel bison'
       - name: Build and test
         run: rake -m test:build && rake test:run
 
@@ -146,7 +148,7 @@ jobs:
     timeout-minutes: 10
     env:
       MRUBY_CONFIG: ci/msvc
-      BISON: C:\ProgramData\chocolatey\lib\winflexbison3\tools\win_bison.exe
+      YACC: C:\ProgramData\chocolatey\lib\winflexbison3\tools\win_bison.exe
     steps:
       - uses: actions/checkout@v2.3.4
       - name: Ruby version

appveyor.yml

@@ -43,5 +43,6 @@ install:
 
 build_script:
   - set MRUBY_CONFIG=ci/msvc
+  - set YACC=C:\ProgramData\chocolatey\lib\winflexbison3\tools\win_bison.exe
   - rake -m test:build
   - rake test:run