Add shell: cmd

Use cmd instead of new default shell for `call`.
https://github.blog/changelog/2019-10-17-github-actions-default-shell-on-windows-runners-is-changing-to-powershell/

https://github.com/ruby/ruby/commit/7bcc639c720b59eb2199601963bcfa720168185b/checks#step:6:5
```
call : The term 'call' is not recognized as the name of a cmdlet, function, script file, or operable program.
```

.github/workflows/windows.yml

@@ -43,16 +43,19 @@ jobs:
         run: |
           call "C:\Program Files (x86)\Microsoft Visual Studio\${{ matrix.vs }}\Enterprise\VC\Auxiliary\Build\vcvars64.bat"
           win32/configure.bat --disable-install-doc --without-ext=+,dbm,gdbm --enable-bundled-libffi --with-opt-dir=C:/vcpkg/installed/x64-windows
+        shell: cmd
       - name: nmake
         run: |
           call "C:\Program Files (x86)\Microsoft Visual Studio\${{ matrix.vs }}\Enterprise\VC\Auxiliary\Build\vcvars64.bat"
           set YACC=win_bison
           nmake up
           nmake
+        shell: cmd
       - name: nmake test
         run: |
           call "C:\Program Files (x86)\Microsoft Visual Studio\${{ matrix.vs }}\Enterprise\VC\Auxiliary\Build\vcvars64.bat"
           nmake ${{ matrix.test_task }}
+        shell: cmd
       - uses: k0kubun/action-slack@v2.0.0
         with:
           payload: |