matrix.yml: exercise architecture: ucrt64 with flavor: full

The manually-triggered `test all artifact flavors` workflow is the
closest thing this repository has to an end-to-end smoke test that
actually downloads an SDK and runs in-place. Without an entry for the
new `ucrt64` axis it would silently keep verifying only the MINGW64
and MINGW32 paths, leaving regressions on the UCRT64 path to surface
only in consumer repositories.

Add a single `flavor: full, architecture: ucrt64` row via `include`,
because that is the only combination wired up end-to-end at this
point: the subset flavors still depend on `please.sh create-sdk-artifact`
in `build-extra` learning `--architecture=ucrt64` and on a UCRT64
asset appearing in the `ci-artifacts` release of `git-sdk-64`, both
tracked in
git-for-windows/git-sdk-64#117 (comment).
The build-installer step keys off `matrix.flavor == 'build-installers'`
and is therefore skipped for the new row, which is exactly what we
want.

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

Author: dscho

.github/workflows/matrix.yml

@@ -15,6 +15,13 @@ jobs:
             architecture: i686
           - flavor: makepkg-git
             architecture: i686
+        # `ucrt64` is wired up end-to-end only for `flavor: full`; the
+        # subset flavors depend on follow-up work in `build-extra` and
+        # the `ci-artifacts` pipeline of `git-sdk-64`. Add the one
+        # combination we can meaningfully exercise.
+        include:
+          - flavor: full
+            architecture: ucrt64
     steps:
       - uses: actions/checkout@v6
       - name: run in-place