[ci] Add stats comment with partial stats (#95401)
We used to not post a comment when a part of the bundler matrix got
cancelled. Conceptually, a cancelled job is just like a failed on in the
sense of not being able to produce stats.
Now we always aggregate stats and post stats from the bundlers that did
produce them.
The risk here is a race between a run that was cancelled because another
run superseded it and the run that superseded it. Though practically,
aggregating the stats isn't slower than running the full workflow. Not
impossible (especially given how slow the Webpack job is) but incredibly
rare.
