We were already using the latest released version of this action, but it
runs shellcheck on files ending in `.zsh`. Shellcheck does not support
zsh, so this produces an error.

The latest release of the action only supports ignoring directories, not
files. And we shouldn't ignore the completions directory, because we
want to run shellcheck on the `configlet.bash` completion script.

For now, it looks like the best solution is to bump to the latest
ref (2021-11-14). This includes an upstream commit [1] that is supposed
to exclude `.zsh` files, but it doesn't seem to work. We need to use the
`ignore_names` input that does not exist in the latest release.

Running shellcheck on `configlet.zsh` produces these errors:

    In completions/configlet.zsh line 1:
    #compdef configlet
    ^-- SC2148 (error): Tips depend on target shell and yours is unknown. Add a shebang or a 'shell' directive.

    In completions/configlet.zsh line 25:
      words=($line[1] "${words[@]}")
             ^-- SC1087 (error): Use braces when expanding arrays, e.g. ${array[idx]} (or ${var}[.. to quiet).
             ^---^ SC2206 (warning): Quote to prevent word splitting/globbing, or split robustly with mapfile or read -a.

    In completions/configlet.zsh line 27:
      curcontext="${curcontext%:*:*}:configlet-command-$line[1]:"
                                                       ^-- SC1087 (error): Use braces when expanding arrays, e.g. ${array[idx]} (or ${var}[.. to quiet).

    In completions/configlet.zsh line 28:
      case $line[1] in
           ^-- SC1087 (error): Use braces when expanding arrays, e.g. ${array[idx]} (or ${var}[.. to quiet).

    In completions/configlet.zsh line 95:
    (( $+functions[_configlet_commands] )) ||
         ^----------------------------^ SC2154 (warning): functions is referenced but not assigned.
                   ^-----------------^ SC2154 (warning): _configlet_commands is referenced but not assigned.

    In completions/configlet.zsh line 98:
      commands=(
      ^------^ SC2034 (warning): commands appears unused. Verify use (or export if used externally).

    For more information:
      https://www.shellcheck.net/wiki/SC1087 -- Use braces when expanding arrays,...
      https://www.shellcheck.net/wiki/SC2148 -- Tips depend on target shell and y...
      https://www.shellcheck.net/wiki/SC2034 -- commands appears unused. Verify u...

[1] ludeeus/action-shellcheck@ceeca77f6538

Refactor later.