[openstreetmap/openstreetmap-website] Stop GitHub form cancelling related workflows early (PR #6151)

[Marwin Hochfelsner]

hlfan left a comment (openstreetmap/openstreetmap-website#6151)

Let's take a look at the latest test run that didn't fail on "real bugs", [this test run](https://github.com/openstreetmap/openstreetmap-website/actions/runs/16122953585) for #6170:

The three executions of `rails test:all` finished in 297.341805s, 281.573575s, and 298.088535s.
Once this step finishes, the remainder of the workflow usually takes less than 15 seconds.

Now, one Ruby run fails:
- Either the other runs can complete, costing about 20 seconds of compute time plus the re-run of one job.
- Or this run cancels the other runs, costing the re-run of two jobs.

Unless you can make the jobs take less than 20 seconds, the current variant takes more time in this example.

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Due to the usually parallel execution of the test runs, unless the error happens within the first two minutes of the workflow during the setup, `rails test:all` will only report an error after about seven minutes into the run, when the other runs are also almost complete.
Thus, the "wasted [compute] time running tests that need to be rerun" will be bigger **with** `fail-fast`, **unless** test-caused (not bug-caused) failures occur more often than once in every 21 runs — based on the fact that cancelling other jobs saves only about 20 seconds per job, while letting them finish typically costs around 7 minutes total.

And currently, I don't see test failures being even close to the necessary 95.2% bug-caused failure rate.
Therefore, `fail-fast` currently **increases** total compute time, because most jobs are cancelled _just seconds before completion_, and have to be re-run anyway. So we end up paying twice.

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