ci-artifacts: build git-for-windows/git, not git/git, for the UCRT64 SDK

Cloning git/git's `master` no longer compiles against this branch's
SDK: with `MSYSTEM=UCRT64`, `config.mak.uname` upstream falls into the
catch-all `else` of its MSYSTEM ladder (MINGW32 / MINGW64 / CLANGARM64)
and appends `-D_USE_32BIT_TIME_T` to `COMPAT_CFLAGS`, which UCRT then
rejects with `#error You cannot use 32-bit time_t (_USE_32BIT_TIME_T)
with _WIN64`.

The git-for-windows fork rewrote that block to derive `MINGW_PREFIX`
and `HOST_CPU` from the MSYS2-provided variables, so UCRT64 just
works, the same way it does for the `mingw-w64-git` PKGBUILD that
sources from this fork. Switching the ci-artifacts smoke build to the
fork's `main` is also a closer match to what we actually ship.

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

Author: dscho

.github/workflows/ci-artifacts.yml

@@ -44,9 +44,12 @@ jobs:
         with:
           name: minimal-sdk
           path: git-sdk-x86_64-minimal.tar.gz
-      - name: clone git.git's `master`
-        run: git clone --depth=1 --branch master https://github.com/git/git ..\git
-      - name: build current `master` of git.git
+      - name: clone git-for-windows/git's `main`
+        shell: bash
+        run: |
+          git clone --depth=1 --branch main \
+            https://github.com/git-for-windows/git ..\git
+      - name: build current `main` of git-for-windows/git
         shell: bash
         run: |
           set -x