GitHub workflow: automatically follow minor updates of setup-msbuild

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 <redacted>
Signed-off-by: Junio C Hamano <redacted>