Work around Windows CI failures

[khaeru]

This PR continues from the issue discovered at #452 (comment):

The Windows tests appear to all pass, but the step is failing:

XFAIL message_ix/tests/test_core.py::test_add_horizon[args7-kwargs7-None]
XFAIL message_ix/tests/test_core.py::test_add_horizon[args8-kwargs8-None]
==== 113 passed, 2 deselected, 5 xfailed, 4 warnings in 444.20s (0:07:24) =====
Error: Process completed with exit code 1.

Steps taken:

• Re-enable Windows CI.
• File Pytest gives exit code 127 pytest-dev/pytest#8510 and discuss with pytest devs.
• May also file a bug in GitHub's virtual-environments repo.
• Introduce a workaround in the "pytest" CI workflow. This is not a genuine fix for the issue, but we lack the resources (time) to efficiently debug and resolve it.
• File Resolve Windows access violations in JDBCBackend ixmp#411.

How to review

No review; CI changes only.

PR checklist

• Continuous integration checks all ✅
• Add or expand tests; coverage checks both ✅
• Add, expand, or update documentation. N/A; CI changes only.
• Update release notes.

[codecov]

Codecov Report

Merging #453 (b3f4580) into master (4930f90) will not change coverage.
The diff coverage is n/a.

❗ Current head b3f4580 differs from pull request most recent head d76d76c. Consider uploading reports for the commit d76d76c to get more accurate results

@@          Coverage Diff           @@
##           master    #453   +/-   ##
======================================
  Coverage    93.3%   93.3%           
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  Files          37      37           
  Lines        2800    2800           
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  Hits         2613    2613           
  Misses        187     187