Allow to include shell options in CIRRUS_SHELL

[Jackenmen]

Right now, CIRRUS_SHELL can only contain the path to the shell executable, it cannot include any arguments after it. Therefore I cannot use something like:

env:
  CIRRUS_SHELL: /bin/bash -l

to get bash to act as if it had been invoked as a login shell.

This is available in some of the other CI solutions:

• https://docs.github.com/en/actions/using-workflows/workflow-syntax-for-github-actions#jobsjob_idstepsshell
• https://circleci.com/docs/2.0/configuration-reference/#default-shell-options