[ci] CPU integration tests are bottleneck-ing CI

[driazati]

The CI runtime has creeped up to around 2.5 hours from 1.8~ish. Looking at https://monitoring.tlcpack.ai/d/hB2FYR87k/build-status?orgId=1, this happened this week after #12030 merged. The second chart shows that the CPU integration steps are taking far longer than the others:

We should increase the CPU integration test shards (probably from 6 to around 10) to keep runtime down.

cc @Mousius @areusch

[areusch]

cc @hpanda-naut for triage

@mikepapadim would you like to take a look at this one?

[mikepapadim]

I am having a look into this.

[mikepapadim]

this should be done with #12316