GHA/windows: switch a dl-mingw job to skeeto/w64devkit gcc 15.1.0

To add another, so far untested standalone toolchain variant to the mix.
This distro is a fairly compact, GCC mingw-w64.

Replacing an existing 15.0.1 snapshot toolchain build job.

Ref: https://github.com/skeeto/w64devkit/releases

Closes #19369

.github/workflows/windows.yml

@@ -418,14 +418,16 @@ jobs:
       matrix:
         include:
           - name: 'schannel'  # mingw-w64 12.0
-            dir: 'mingw64'
+            sys: 'mingw64'
+            dir: 'w64devkit'
             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'
+            ver: '15.1.0'
+            url: 'https://github.com/skeeto/w64devkit/releases/download/v2.2.0/w64devkit-x64-2.2.0.7z.exe'
             config: '-DENABLE_DEBUG=ON -DBUILD_SHARED_LIBS=OFF -DCURL_USE_SCHANNEL=ON -DENABLE_UNICODE=OFF'
             type: 'Release'
             tflags: 'skiprun'
           - name: 'schannel'  # mingw-w64 10.0
+            sys: 'mingw64'
             dir: 'mingw64'
             env: 'x86_64'
             ver: '9.5.0'
@@ -433,6 +435,7 @@ jobs:
             config: '-DENABLE_DEBUG=ON -DBUILD_SHARED_LIBS=OFF -DCURL_USE_SCHANNEL=ON -DENABLE_UNICODE=OFF'
             type: 'Release'
           - name: 'schannel mbedtls U'  # mingw-w64 6.0
+            sys: 'mingw64'
             dir: 'mingw64'
             env: 'x86_64'
             ver: '7.3.0'
@@ -442,6 +445,7 @@ jobs:
             type: 'Release'
             tflags: 'skiprun'
           - name: 'schannel !unity'  # mingw-w64 5.0
+            sys: 'mingw32'
             dir: 'mingw32'
             env: 'i686'
             ver: '6.4.0'
@@ -450,6 +454,7 @@ jobs:
             type: 'Debug'
             tflags: 'skiprun'
           - name: 'schannel !examples'  # mingw-w64 3.0
+            sys: 'mingw64'
             dir: 'mingw64'
             env: 'x86_64'
             ver: '4.8.1'
@@ -461,7 +466,7 @@ jobs:
     steps:
       - uses: msys2/setup-msys2@fb197b72ce45fb24f17bf3f807a388985654d1f2 # v2.29.0
         with:
-          msystem: ${{ matrix.dir }}
+          msystem: ${{ matrix.sys }}
           release: false
           update: false
           cache: false