incompatible_run_shell_command_string: Disallow "command" parameter of actions.run_shell to take a sequence

[c-parsons]

This tracks removal of a long-deprecated construct of the command parameter of ctx.actions.run_shell and ctx.actions.run accepting a sequence of strings.

[laurentlb]

The goal of ctx.actions.run_shell is to run a command using a shell. A shell command is a string, not a list.

    ctx.actions.run_shell(
        outputs = [f],
        command = "touch " + f.path,
    )

If a user wants to run a command with an argument, e.g. "touch output" they can use ctx.actions.run:

    ctx.actions.run(
        outputs = [f],
        executable = "touch",
        arguments = [f.path],
    )

So I think we should require command to be a string. Fixing existing code should be simple using ctx.actions.run.

[laurentlb]

Increasing to P1, assuming we still want to do this breaking change.