Add 3.1.0 Windows to CI

[eregon]

[MSP-Greg]

I thought 'might as well check things with ruby-spec', and saw this. I changed the CI to run on all six OS's, ran one with setup-ruby, and one with my fork's PR branch. Don't really have time to look at it right now.

setup-ruby: https://github.com/MSP-Greg/spec/actions/runs/1680464347 PR branch: https://github.com/MSP-Greg/spec/actions/runs/1680459998

The error in my branch with Windows Ruby 3.1.0 (ConstantLeakError: Top level constants leaked in the whole test suite: Fiddle) may have something to do with a change to the build, as it no longer loads Fiddle with rubygems/defaults/operating_system.rb, Not.Sure.

The odd thing is the failures with Ubuntu-18.04. Again, not sure, but I thought I'd mention it here...

[eregon]

Thanks. I'm not sure what loads Fiddle, but either way that happens in both cases and is unlikely to be ucrt-or-not related. We can just add Fiddle to .mspec.constants.

Your run (on windows 2019) sees the same non-Fiddle error as I got in this PR:

1)
C-API String function RSTRING_PTR allows read() to update the string contents ERROR
Errno::NOERROR: No error - read
D:/a/spec/spec/optional/capi/string_spec.rb:605:in `RSTRING_PTR_read'
D:/a/spec/spec/optional/capi/string_spec.rb:605:in `block (3 levels) in <top (required)>'
D:/a/spec/spec/optional/capi/string_spec.rb:42:in `<top (required)>'

3)
C-API Kernel function rb_sys_fail can take a NULL message ERROR
Expected Errno::EPERM
but got: Errno::ECHILD (No child processes)
D:/a/spec/spec/optional/capi/kernel_spec.rb:166:in `rb_sys_fail'
D:/a/spec/spec/optional/capi/kernel_spec.rb:166:in `block (4 levels) in <top (required)>'
D:/a/spec/spec/optional/capi/kernel_spec.rb:165:in `block (3 levels) in <top (required)>'
D:/a/spec/spec/optional/capi/kernel_spec.rb:5:in `<top (required)>'

So it seems the proper ucrt setup does fix those.

[eregon]

Merged in https://github.com/ruby/spec/commit/82cd3a3583e245f5ee4430b39ea560a419218347