Disable windows_portable builds

So they don't run and fail in nighly builds, see https://github.com/Jojo-Schmitz/MuseScore/actions/runs/12489274000/job/34852690927

.github/workflows/build_windows.yml

@@ -14,7 +14,7 @@ on:
     inputs:
       platforms:
         description: 'Platforms to build:'
-        default: 'windows_x64 windows_x32 windows_portable'
+        default: 'windows_x64 windows_x32' # no windows_portable :-(
         required: true
       build_mode:
         description: 'Build mode: devel, nightly, testing, stable'
@@ -28,7 +28,7 @@ on:
     inputs:
       platforms:
         description: 'Platforms to build:'
-        default: 'windows_x64 windows_x32 windows_portable'
+        default: 'windows_x64 windows_x32' # no windows_portable :-(
         type: string
         required: true
       build_mode: