feat: add the ability to override the default shell used by shelljs.exec

[shetzel]

This adds the TESTKIT_EXEC_SHELL env var which allows tests to specify a shell other than the default used by shelljs, which is the same default as child_process. This shell will be used everywhere shelljs.exec() is called. It will still be possible to use the env var to override the shell but use a specific one when calling execCmd().

This affect:
execCmd()
TestSession setupCommands
TestProject creation with git clone or project:create

@W-8996057@

[peternhale]

@shetzel will there be a change to the orb to set the env var on windows VMs only?

[shetzel]

@shetzel will there be a change to the orb to set the env var on windows VMs only?

If a different shell is needed per job then something in Circle CI config will need to change. Maybe that's the orb or maybe it's the project's circle config? I'm not sure.