.github/workflows/main.yml: force mingw32 test to Windows 2019.

Actions' windows-latest is equipped with newer mingw, which causes
linking problems in mingw32 debug builds. See #677 for details.

Author: dot-asm

.github/workflows/main.yml

@@ -48,7 +48,7 @@ jobs:
             rust: stable
             target: x86_64-pc-windows-msvc
           - build: mingw32
-            os: windows-latest
+            os: windows-2019
             rust: stable-i686-gnu
             target: i686-pc-windows-gnu
           - build: mingw64