win32: Use right name for GIO_USE_VOLUME_MONITOR in CI

testsuite/meson.build

@@ -9,7 +9,6 @@ if catch.found()
 endif
 
 common_env = [
-  'GIO_USE_VOLUME_MONITOR=unix',
   'GIO_USE_VFS=local',
   'GSETTINGS_BACKEND=memory',
   'GTK_CSD=1',
@@ -20,6 +19,13 @@ common_env = [
 ]
 exclude_unstable = ['flaky', 'failing']
 
+if os_win32
+  common_env += [ 'GIO_USE_VOLUME_MONITOR=win32' ]
+else
+  common_env += [ 'GIO_USE_VOLUME_MONITOR=unix' ]
+endif
+
+
 setups = [
   { 'backend': 'x11', 'if': x11_enabled, },
   { 'backend': 'wayland', 'if': wayland_enabled, 'is_default': true, },