Ensure that bazelisk is present.

If bazelisk is already installed, this role does nothing.  Otherwise,
it downloads bazelisk from GitHub and installs it in the user's
home directory by default.

**Role Variables**

.. zuul:rolevar:: bazelisk_version
   :default: v1.3.0

   Version of bazelisk to install.

.. zuul:rolevar:: bazelisk_arch
   :default: linux-amd64

   Architecture to install.

.. zuul:rolevar:: bazelisk_url
   :default: https://github.com/bazelbuild/bazelisk/releases/download/{{ bazelisk_version }}/bazelisk-{{ bazelisk_arch }}

   The URL from which to download bazelisk.

.. zuul:rolevar:: bazelisk_target
   :default: "{{ ansible_user_dir }}/.local/bin/bazelisk"

   Where to install bazelisk.  If the role downloads bazelisk, it will
   set :zuul:rolevar:`ensure-bazelisk.bazelisk_executable` to this value as well.

**Output Variables**

.. zuul:rolevar:: bazelisk_executable
   :default: bazelisk

   The bazelisk executable.  If this already exists, the role will not
   perform any further actions.  It will be made available for other
   roles running after role.