ci(github): make Windows test artifacts name unique

ci(github): make Windows test artifacts name unique

If several jobs in the windows-test or vs-test matrices fail, the
upload-artifact action in each job tries to upload the test directories
of the failed tests as "failed-tests-windows.zip", which fails for all
jobs except the one which finishes first with the following error:

    Error: Failed to CreateArtifact: Received non-retryable error:
    Failed request: (409) Conflict: an artifact with this name
    already exists on the workflow run

Make the artifacts name unique by using the 'matrix.nr' token, and
disambiguate the vs-test artifacts from the windows-test ones.

Signed-off-by: Philippe Blain <levraiphilippeblain@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>