ci(workflow): store plain text test summary (#3567)
Requires https://github.com/vercel/turbo/pull/3534
Partially progresses WEB-460
This PR creates a plain text summary, so subsequent workflow can
download it via download-artifact then broadcast as needed. We'll use it
to posting into Slack channel. Action will only generate text file when
we supposed to report (currently, release-to-release) so if
download-artifact does not contain text-summary.md, it can skip to
broadcast into Slack channel.
