Add step to fetch configlet to PR template

[muzimuzhi]

Summary

Program configlet mentioned in the Checklist of PR template is not contained in this repo. This PR adds the step to fetch the latest configlet.

I noticed this repo also contains a fetch-configlet.ps1 for Windows PowerShell users. Is it recommended to also add a variant for Windows users?

TODO: Sync the change to https://github.com/exercism/common-lisp, from where the .github/pull_request_template.md is copied. See #106.

Checklist

• If docs where changed, run ./bin/configlet generate to ensure all documents are properly generated.
• CI is green

[glennj]

The fetch* scripts are managed centrally and pushed out to all track repos.

I think windows users can probably figure it out. If not, they'll let us know.

[muzimuzhi]

The fetch* scripts are managed centrally and pushed out to all track repos.

True for fetch* scripts (found it in https://github.com/exercism/org-wide-files/tree/main/tracks-files/bin), but I meant to refer to the PR template, which is not managed centrally and is only contained in a few tracks (currently only 4), with different contents.
https://github.com/search?q=org%3Aexercism+path%3A%22.github%2Fpull_request_template.md%22&type=code

Do you suggest adding a similar ./bin/fetch-configlet to the PR template in common-lisp track?