Stop cancelling superseded runs on main

[albertvillanova]

What does this PR do?

#7172 grouped push runs by github.ref so that a newer merge cancels the run started by the previous one. It saves the runner time it claimed, but it makes main look broken: I thought it was going to be grey instead of red.

GitHub rolls a commit's checks up into the single icon shown in the commit list, and it buckets cancelled with failure. The three most recent merges before this PR:

FAILURE  cancelled=5 failure=0   Move the latest release tests from T4 to L40S (#7184)
FAILURE  cancelled=5 failure=0   Use the canonical id for the tiny GPTNeoX model (#7185)
FAILURE  cancelled=0 failure=1   Train the tiny DeepSeek-R1-Distill model (#7187)

The first two are red for nothing. The third is a real failure, and it is now indistinguishable from the noise. Since #7172 merged, 3 of the 8 commits on main carry a red rollup and 2 of them only because their runs were cancelled. A history where most commits show a failing icon teaches everyone to stop reading the icon, which costs more than the runner time the change saved.

There is no way to recolour it. Only success, skipped and neutral avoid red; a workflow job cannot report neutral for itself, and a job-level if: is evaluated when the job is scheduled, which is before we can know that a later commit is coming.

This restores github.run_id as the fallback, so every push gets its own group again. Cancellation of superseded pull request runs, the larger half of #7170 and the part that was not in question, is unchanged. The !cancelled() guard added to the Slack steps in #7172 stays and simply becomes inert.

Applied to tests-experimental.yml as well so the two blocks stay identical.

Removing redundant main runs without attaching cancelled checks to commits needs a mechanism that tests batches before they land rather than after, which is what the GitHub merge queue does. That is a bigger change and a separate conversation.

Part of #7169.

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Note

Low Risk
CI workflow-only change; restores per-push runs on main without altering test logic or PR cancel-in-progress behavior.

Overview
Reverts the non-PR GitHub Actions concurrency key from github.ref back to github.run_id in tests.yml and tests-experimental.yml, with the same one-line change in both workflows.

On main pushes, each run now gets its own concurrency group again, so a newer merge no longer cancels the previous push’s jobs. That avoids cancelled workflows rolling up as red/failed on the commit list when nothing actually broke. Pull request behaviour is unchanged: the group still uses github.event.pull_request.number, so superseded PR runs can still be cancelled via cancel-in-progress: true.

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[albertvillanova]

Merging this PR as a CI hotfix.