Run system:msvc:windows:amd64 for merge requests

Historically, we avoided running system tests on Windows for merge request-triggered pipelines because we have only one limited GitLab Runner, which would be unable to reliably sustain the CI load of merge requests for three maintained branches, and instead, we opted for running Windows system tests in daily pipelines. CI load should no longer be an issue since "v9_16" is the only (and last) maintained branch with Windows support. We are only left with the downside of failing to run system tests for merge requests on Windows: missing Windows-specific quirks in the review process and finding out about them only a day later in the daily pipeline (e.g., #3865 (closed) and #3874 (closed) from this week).