GitHub workflow: automatically follow minor updates of setup-msbuild

It is the custom to follow minor updates of GitHub Actions automatically, by using the suffix `@v1`. Actions' maintainers will then update that `v1` ref to point to the newest. However, for `microsoft/setup-msbuild`, 889cacb6897 (ci: configure GitHub Actions for CI/PR, 2020-04-11) uses a very specific `@v1.0.0` suffix. In this instance, that is a problem: should `setup-msbuild` release a new version that intends to fix a critical bug, we won't know it, and we won't use it. Such a scenario is not theoretical. It is happening right now: https://github.blog/changelog/2020-10-01-github-actions-deprecating-set-env-and-add-path-commands Let's simplify our setup, allowing us to benefit from automatically using the newest v1.x. Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de> Signed-off-by: Junio C Hamano <gitster@pobox.com>