GitHub-CI (MSYS2/MinGW): Build with UCRT64 instead of CLANG64.

* .github/workflows/make.yaml (mingw): Run CI in the UCRT64 environment instead
of in the CLANG64 environment. That is closer to what will likely distributed
as version 9 of Octave for Windows.

.github/workflows/make.yaml

@@ -672,7 +672,7 @@ jobs:
       matrix:
         # For available GitHub-hosted runners, see: https://docs.github.com/en/actions/using-github-hosted-runners/about-github-hosted-runners
         os: [windows-latest]
-        msystem: [MINGW64, CLANG64]
+        msystem: [MINGW64, UCRT64]
         include:
           - msystem: MINGW64
             mingw-prefix: mingw64
@@ -683,20 +683,12 @@ jobs:
             extra-config-flags: ""
             ccache-max: 0.9G
           - msystem: CLANG64
-            mingw-prefix: clang64
-            target-prefix: mingw-w64-clang-x86_64
-            cc: clang
-            # It looks like we and graphicsmagick++ aren't ready for C++17 yet.
-            cxx: "clang++ -std=gnu++14"
-            f77: flang
-            # Clang seems to require a different set of dllexport attributes than GCC.
-            # autoconf and libtool still need help to correctly invoke flang.
-            extra-config-flags:
-              --disable-lib-visibility-flags
-              --enable-fortran-calling-convention=gfortran
-              ac_cv_f77_compiler_gnu=yes
-              lt_cv_prog_gnu_ld=yes
-            ccache-max: 400M
+            mingw-prefix: ucrt64
+            target-prefix: mingw-w64-ucrt-x86_64
+            cc: gcc
+            cxx: g++
+            f77: gfortran
+            ccache-max: 0.9G
 
     env:
       CHERE_INVOKING: 1