Run with --jobs=2 automatically on CI

[soutaro]

Steep invokes worker processes to type check files as many as the processors. But, computers running CI jobs typically have large number of processors, while the resources each CI job can use is limited.

Assume the CI computer has 32 cores, but only two of them are available for CI job. Steep spawns ~32 worker processes based on the number of cores. The problem is it often hits the memory limit by starting too many workers.

This PR is to fix the problem by setting the default --jobs number to 2 (or # of cores if it's smaller) when:

1. CI environment variable is defined, and
2. --jobs option is not set

This can slow the type checking job during CI if more than two cores are available. In that case, specify --jobs option explicitly based on the number of the available cores.

Another possible implementation

This is kind of a workaround and calculating actually available number of processors would be a better solution for this problem. This is left for future work. 😃