test.yml: exercise the ucrt64 subset flavors on every PR

The per-PR `test` matrix already covered `ucrt64` + `full` end-to-end.
Now that `please.sh create-sdk-artifact --architecture=ucrt64` works,
extend the matrix with the three subset flavors so a regression on
the `getViaGit` -> `please.sh` path for `ucrt64` is caught on every
PR rather than first surfacing in a consumer repository.

`mingw_prefix` stays at `/ucrt64` for all three new rows; the
verification step's `$MINGW_PREFIX/bin/gcc` assertion checks the
same path on every flavor, since `gcc` is present in `minimal`,
`makepkg-git`, `build-installers`, and `full` alike.

Also drop the now-stale comment explaining why `ucrt64` only had a
`full` row.

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

Author: dscho

.github/workflows/test.yml

@@ -45,11 +45,15 @@ jobs:
           - architecture: x86_64
             flavor: minimal
             mingw_prefix: /mingw64
-          # `ucrt64` is wired up end-to-end only for `flavor: full` at
-          # this point (the subset flavors depend on follow-up work in
-          # `build-extra` and the `ci-artifacts` pipeline of
-          # `git-sdk-64`), so that is the one combination we can
-          # meaningfully exercise on every PR.
+          - architecture: ucrt64
+            flavor: minimal
+            mingw_prefix: /ucrt64
+          - architecture: ucrt64
+            flavor: makepkg-git
+            mingw_prefix: /ucrt64
+          - architecture: ucrt64
+            flavor: build-installers
+            mingw_prefix: /ucrt64
           - architecture: ucrt64
             flavor: full
             mingw_prefix: /ucrt64