bench: stop manual runs from cancelling each other (#11)

`cancel-in-progress: true` is right for a CI triggered by pushes, where a newer
commit makes an older run pointless. It is wrong here. Three times in one
afternoon a second dispatch killed a run that was mid-measurement, once eight
minutes in, and each left a cancelled entry in a run list whose whole job is to
be readable by someone checking the numbers.

The fix people reach for then is deleting runs, which breaks the run_url the
published data links to, and with it the only reason to believe the numbers.

A manual run is never superseded: nobody dispatches a benchmark to invalidate
the one already running. A release or the weekly cron still supersedes, because
there an older run really is measuring a version nobody asks about any more.

.github/workflows/bench-vs-vectorbt.yml

@@ -38,7 +38,16 @@ permissions:
 
 concurrency:
   group: bench-vs-vectorbt-${{ github.ref }}
-  cancel-in-progress: true
+  # A manual run is never superseded, so it is never cancelled. Cancelling in
+  # flight is the right default for a CI triggered by pushes, where a newer
+  # commit makes an older run pointless; here it threw away ten minutes of
+  # measurement because somebody pressed the button twice, three times in one
+  # afternoon, and left a trail of cancelled runs in a list whose whole job is
+  # to be readable by someone checking the numbers.
+  #
+  # A release or the weekly cron still supersedes: there, an older run really is
+  # measuring a version nobody is asking about any more.
+  cancel-in-progress: ${{ github.event_name != 'workflow_dispatch' }}
 
 jobs:
   bench: