CI: Switch from MINGW64 to UCRT64. Closes: #1421

UCRT64 is meanwhile the recommended MSYS2 environment for Windows.
Visual Studio uses it by default since 2015. tmpfile() behaves here
also as expected and does not always return NULL.

See also:
- https://www.msys2.org/docs/environments/
- https://www.msys2.org/news/#2022-10-29-changing-the-default-environment-from-mingw64-to-ucrt6
- msys2/MINGW-packages#18878

.github/actions/install-dependencies/action.yml

@@ -20,13 +20,13 @@ runs:
     - uses: msys2/setup-msys2@v2
       if: startsWith(inputs.os, 'windows')
       with:
-        msystem: MINGW64
+        msystem: UCRT64
         update: true
         install: >-
-          mingw-w64-x86_64-autotools
-          mingw-w64-x86_64-gcc
-          mingw-w64-x86_64-gtk2
-          mingw-w64-x86_64-meson
-          mingw-w64-x86_64-pkg-config
-          mingw-w64-x86_64-qt6-base
-          mingw-w64-x86_64-qt6-svg
+          mingw-w64-ucrt-x86_64-autotools
+          mingw-w64-ucrt-x86_64-gcc
+          mingw-w64-ucrt-x86_64-gtk2
+          mingw-w64-ucrt-x86_64-meson
+          mingw-w64-ucrt-x86_64-pkg-config
+          mingw-w64-ucrt-x86_64-qt6-base
+          mingw-w64-ucrt-x86_64-qt6-svg