Avoid using git restore-mtime

It's failing with:

'git whatchanged' is nominated for removal.

hint: You can replace 'git whatchanged <opts>' with:
hint:	git log <opts> --raw --no-merges
hint: Or make an alias:
hint:	git config set --global alias.whatchanged 'log --raw --no-merges'

If you still use this command, here's what you can do:

- read https://git-scm.com/docs/BreakingChanges.html
- check if anyone has discussed this on the mailing
  list and if they came up with something that can
  help you: https://lore.kernel.org/git/?q=git%20whatchanged
- send an email to <git@vger.kernel.org> to let us
  know that you still use this command and were unable
  to determine a suitable replacement

fatal: refusing to run without --i-still-use-this

Author: gsmet

.github/workflows/build.yml

@@ -22,11 +22,9 @@ jobs:
         with:
           ruby-version: 3.2.3
 
-      - name: Install git-restore-time
-        run: sudo apt-get install -y git-restore-mtime
-
+      # git restore-mtime is currently broken in the GH Actions images
       - name: Restore mtime
-        run: git restore-mtime
+        run: git ls-files -z | while read -d '' path; do touch -d $(git log -1 --format="@%ct" "$path") "$path"; done
 
       - name: Build Jekyll site
         uses: limjh16/jekyll-action-ts@v2