fix workflow 'test'

[eine]

This PR is based on #18

In the last execution of master, job set works as expected: https://github.com/numworks/setup-msys2/runs/271213127#step:7:14. However, in the latest PRs, it does not:

• https://github.com/numworks/setup-msys2/pull/15/checks?check_run_id=322337129#step:7:20
  https://github.com/numworks/setup-msys2/pull/14/checks?check_run_id=322323700#step:7:25

This is because the default shell in windows-latest changed in the last month from cmd to powershell:

• https://github.blog/changelog/2019-10-17-github-actions-default-shell-on-windows-runners-is-changing-to-powershell/
• https://help.github.com/en/actions/automating-your-workflow-with-github-actions/workflow-syntax-for-github-actions#using-a-specific-shell

In this PR, the shell is specified, so that set is properly executed. Furthermore, two similar jobs are added: one to use powershell syntax ($env:MSYSTEM = 'MINGW32') and another one to set the envvars in the env fields of the workflow.