GHA/windows: add gcc-15 job

It's taking 2.5 minutes and planned for removal when MSYS2 gcc-15 gets
deployed in CI.

15.0.1 builds significantly faster than 9.5.0. (But still slower than
7.3.0 and 6.4.0)

Ref: f59921184b
Ref: https://github.com/msys2/MINGW-packages/pull/24037

Closes #17190

.github/workflows/windows.yml

@@ -399,6 +399,14 @@ jobs:
     strategy:
       matrix:
         include:
+          - name: 'schannel'
+            dir: 'mingw64'
+            env: 'x86_64'
+            ver: '15.0.1'
+            url: 'https://github.com/brechtsanders/winlibs_mingw/releases/download/15.0.1-snapshot20250406posix-12.0.0-ucrt-r1/winlibs-x86_64-posix-seh-gcc-15.0.1-snapshot20250406-mingw-w64ucrt-12.0.0-r1.7z'
+            config: '-DENABLE_DEBUG=ON -DBUILD_SHARED_LIBS=OFF -DCURL_USE_SCHANNEL=ON -DENABLE_UNICODE=OFF'
+            type: 'Release'
+            tflags: 'skiprun'
           - name: 'schannel'
             dir: 'mingw64'
             env: 'x86_64'