ci: use the new GitHub Action to download git-sdk-64-minimal

In our continuous builds, Windows is the odd cookie that requires a complete development environment to be downloaded because there is no suitable one installed by default on Windows. Side note: technically, there _is_ a development environment present in GitHub Actions' build agents: MSYS2. But it differs from Git for Windows' SDK in subtle points, unfortunately enough so to prevent Git's test suite from running without failures. Traditionally, we support downloading this environment (which we nicknamed `git-sdk-64-minimal`) via a PowerShell scriptlet that accesses the build artifacts of a dedicated Azure Pipeline (which packages a tiny subset of the full Git for Windows SDK, containing just enough to build Git and run its test suite). This PowerShell script is unfortunately not very robust and sometimes fails due to network issues. Of course, we could add code to detect that situation, wait a little, try again, if it fails again wait a little longer, lather, rinse and repeat. Instead of doing all of this in Git's own `.github/workflows/`, though, let's offload this logic to the new GitHub Action at https://github.com/marketplace/actions/setup-git-for-windows-sdk This Action not only downloads and extracts git-sdk-64-minimal _outside_ the worktree (making it no longer necessary to meddle with `.gitignore` or `.git/info/exclude`), it also adds the `bash.exe` to the `PATH` and sets the environment variable `MSYSTEM` (an implementation detail that Git's workflow should never have needed to know about). This allows us to convert all those funny PowerShell tasks that wanted to call git-sdk-64-minimal's `bash.exe`: they all are now regular `bash` scriptlets. This finally lets us get rid of the funny quoting and escaping where we had to pay attention not only to quote and escape the Bash scriptlets properly, but also to add a second level of escaping (with backslashes for double quotes and backticks for dollar signs) to stop PowerShell from doing unintended things. Further, this Action uses a fast caching strategy native to GitHub Actions that should accelerate the download across CI runs: git-sdk-64-minimal is usually updated once per 24h, and needs to be cached only once within that period. Caching it (unfortunately only on a per-branch basis) speeds up the download step, and makes it much more robust at the same time by virtue of accessing a cache location that is closer in the network topology. With this we can drop the home-rolled caching where we try to accelerate the test phase by uploading git-sdk-64-minimal as a workflow artifact after using it to build Git, and then download it as workflow artifact in the test phase. Even better: the `vs-test` job no longer needs to depend on the `windows-build` job. The only reason it depended on it was to ensure that the `git-sdk-64-minimal` workflow artifact was available. Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de> Signed-off-by: Junio C Hamano <gitster@pobox.com>