don't run codspeed CI job for every PR

[d-v-b]

Got this email, presumably because the stuff it says is true:

Hello,

You have exceeded the limit of 600 minutes of Macro Runners usage for the organization zarr-developers on the Free plan.
Macro Runners will be disabled until January 31, 2026 when the limit will be reset.

To keep using Macro Runners, you can upgrade your plan by visiting your settings page: https://codspeed.io/settings/organizations/zarr-developers/billing.

Thanks,
The CodSpeed team

We should make the codspeed CI runs opt-in instead of running them automatically.

[d-v-b]

closed by #3673