Re: [PATCH v3 1/7] ci: use the new GitHub Action to download git-sdk-64-minimal

> 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.