Build MinGW in the UCRT64 environment

MSYS2 deprecated MINGW64 in favor of UCRT64 and setup-msys2 warns about
its use. Switch the MinGW builds to UCRT64 to silence the warning.

Change-Id: I93bf2d30fc2389e7d2e81da23030dd2a3a501776

.github/workflows/windows.yml

diff --git a/.github/workflows/windows.yml b/.github/workflows/windows.yml
index fbebd49..f704783 100644
--- a/.github/workflows/windows.yml
+++ b/.github/workflows/windows.yml
@@ -15,11 +15,11 @@
       fail-fast: true
       matrix:
         build_type: [Release]
-        sys: [mingw64]
+        sys: [ucrt64]
         lib: [shared, static]
         include:
-          - sys: mingw64
-            env: x86_64
+          - sys: ucrt64
+            env: ucrt-x86_64
 
     steps:
     - uses: actions/checkout@v4
@@ -35,7 +35,7 @@
           mingw-w64-${{matrix.env}}-cmake
           mingw-w64-${{matrix.env}}-eigen3
           mingw-w64-${{matrix.env}}-gcc
-          ${{matrix.sys == 'mingw64' && format('mingw-w64-{0}-metis', matrix.env) || ''}}
+          ${{matrix.sys == 'ucrt64' && format('mingw-w64-{0}-metis', matrix.env) || ''}}
           mingw-w64-${{matrix.env}}-ninja
           mingw-w64-${{matrix.env}}-suitesparse