Health workflow comment concatenation is broken

[natebosch]

We are getting failures like: https://github.com/dart-lang/test/actions/runs/17932516269/job/50992458268?pr=2544

cat: 'single-comments/*/*.md': No such file or directory

Some initial digging:

In a failing run,

Run ls -R single-comments
single-comments:
comment.md

But in a passing run

Run ls -R single-comments
single-comments:
comment-changelog
comment-do-not-submit

single-comments/comment-changelog:
comment.md

single-comments/comment-do-not-submit:
comment.md

It looks like we are impacted by a change in download-artifact in v5:
https://github.com/actions/download-artifact/blob/main/README.md#v5---whats-new

It looks like the comments could be overwriting eachother now without the extra directory to disambiguate the multiple comment.md files. Do we need to be writing the files differently? The download-artifact docs haven't been much direct help.

cc @mosuem