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

"Johannes Schindelin via GitGitGadget" [off-list ref]
writes:

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.

So nice.