ci: use larger Depot Linux runners

[indygreg]

Default runners have 2 CPUs. We can achieve a nice speed-up (but not quite linear) by leveraging runners with more CPUs.

[indygreg]

With the 4 CPU runners (sample size of 1):

• Before: 40m wall; 8971m machine time
• After: 29m wall; 5318m machine time

Perfect scaling of machine time would be 4486m. Since cost scales linearly with CPU count, this change will increase costs by ~18%.

#1022 will help with CPU scaling by making PGO+BOLT run tests in parallel. So probably looking at a 10-15% cost premium after that merges.

IMO this change is worth it since people time costs more than machine time and faster CI enables people to be more productive.

I haven't tested 8 CPU runners thoroughly. But they do yield additional speed-ups.