Fix output.to_stdout_from_any_process when $stdout has been reassigned

[jdelStrother]

(An alternate fix for https://github.com/rspec/rspec-core/pull/3113)

When running under rspec bisect runner, $stdout & $stderr are reassigned to a StringIO, which would cause "can't convert Tempfile into StringIO" when trying to reopen the stream in CaptureStreamToTempfile

Since CaptureStreamToTempfile reopens the stream rather than reassigning it, I think it makes sense to use STDOUT rather than whatever the current value of $stdout is.

(Please feel free to sanity-check that assumption. I've gone round in circles a few too many times on this)

[JonRowe]

I don't have any objections to this approach apart from the fact it breaks our build due to having already redeclared std out, I'm not sure how to fix this yet...

[jdelStrother]

I've pushed a possible fix, where instead of reopening $stderr, it reopens STDERR and reassigns $stderr. I'm not entirely sure it's going to work, but see what you think and I can squash it if desired.

[JonRowe]

Migrated to the monorepo as rspec/rspec#151