In git-for-windows/build-extra#711 I am
preparing the `git-extra` package for the MINGW64 -> UCRT64 migration.
Once that lands, the natural next step is to actually build the UCRT64
variants of the affected packages via this workflow, and for that the
workflow has to know about UCRT64.

The `setup-git-for-windows-sdk` action already accepts `ucrt64` as a
value for its `architecture` input (which installs the UCRT64 variant
of git-sdk-64), so let's reuse the same spelling as a pseudo
architecture here: the runner is still `windows-latest` (UCRT64 is
just an alternative MSYS2 environment on x86_64), `git.exe` is looked
up under `/ucrt64/bin/`, and `MINGW_ARCH=ucrt64` is passed to
`makepkg-mingw`.

The `BUILD_SRC` flag stays empty, just like for `i686` and `aarch64`,
because building the `.src.tar.gz` for the x86_64 variant of a package
(should it also be built) already covers what would otherwise be
produced for ucrt64.

Assisted-by: Opus 4.7
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

In conjunction with the new `ucrt64` pseudo architecture in the
`build-and-deploy` workflow, teach this script that for UCRT64 builds
the MINGW package prefix is `mingw-w64-ucrt-x86_64` (not the literal
`mingw-w64-ucrt64`), and that the resulting packages are deployed
into the `x86_64` subdirectory of the Pacman repository, alongside
the MINGW64 packages.

The latter is so that the existing `git-for-windows-x86_64.db`
database can keep referencing both flavors during the MINGW64 ->
UCRT64 migration; the new `git-for-windows-ucrt64.db` database
introduced in git-for-windows/pacman-repo#4
lives in the same `x86_64` branch of `pacman-repo` for exactly that
reason.

Assisted-by: Opus 4.7
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

I took most of those changes, except the idea to build the MINGW64 and
UCRT64 packages in the same architecture: The long-term plan is to get
rid of this multi-arch building business, and instead build the packages
for a given `MINGW_ARCH` in their respective `git-sdk-*` (but for
historical reasons, I think we'll keep the names `git-sdk-32` for
mingw32, `git-sdk-64` for ucrt64 and `git-sdk-arm64` for clangarm64).

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>