do not mutate num_of_cores

[aishpant]

Refer to comment in commit "use system cpu count to run spatch" .

Mutating variables outside the scope of a function is in general a bad idea.

• You cannot move this function elsewhere now, because its tied up to the environment
• You cannot unit test this function without mocking a global object
• You have another function (or parts of code) that is implicitly depending on this function being called before, but that is not very clear when you read the code.
• Mutation in general makes code harder to understand

Reported by: @jaseemabid.