ci: Windows: Enable 32bit build

.ci/appveyor.yml

@@ -3,14 +3,14 @@ version: 1.1.{build}
 image: Visual Studio 2019
 
 environment:
-  # matrix:
-    MSYSTEM: MINGW64
-    MSYS_ARCH: "x86_64"
-    ARCH: "64bit"
-
-    # - MSYSTEM: MINGW32
-    #   MSYS_ARCH: "i686"
-    #   ARCH: "32bit"
+  matrix:
+    - MSYSTEM: MINGW64
+      MSYS_ARCH: "x86_64"
+      ARCH: "64bit"
+
+    - MSYSTEM: MINGW32
+      MSYS_ARCH: "i686"
+      ARCH: "32bit"
 
 branches:
   only: