Don't suffix Safari GH Actions artifacts when there's a single chunk

[gsnedders]

This allows us to match the regexp that wpt.fyi uses to assign the
pr_base/pr_head labels.

[gsnedders]

This has failed: https://github.com/web-platform-tests/wpt/actions/runs/15504103111

[gsnedders]

https://staging.wpt.fyi/runs?max-count=100&pr=53008 shows everywhere else happily uploads an empty wrong, which suggests a wpt.fyi side problem.

[gsnedders]

https://staging.wpt.fyi/runs?max-count=100&pr=53008 shows everywhere else happily uploads an empty wrong, which suggests a wpt.fyi side problem.

But only insofar as we were filtering to safari-preview-affected-tests-without-changes-* which matched nothing.

The other job was matching on safari-preview-affected-tests-*, which safari-preview-affected-tests-without-changes does match.

This arguably shows some problem with the naming scheme insofar as the "with changes" artifact names always match the "without changes" ones.

[gsnedders]

Lots of invalid runs showing on https://wpt.fyi/status, all things like:

Conflicting '/css/css-fonts/generic-family-keywords-001.html' found in the merged report

This is gonna be the same problem — it finds both when finding matching artifacts.

But when it expects artifacts whose names match specific globs, the only way to disambiguate these two will be to merge separate artifacts with @actions/upload-artifact/merge or similar.

This does actually allow us to avoid doing all this magic naming to avoid the suffix, because then we can just merge everything into something with the expected name.

[gsnedders]

I split out the unrelated fix to #53024; closing this following the revert in #53016, and I'll simply re-land the whole thing (hopefully) working.