win32: ensure that the MSVC build doesn't terminate when MinGW fails (#2018)

For `matrix`-strategy builds, GitHub actions will terminate all related
matrix legs when one of them fails.

Disabling `fail-fast` knocks this behavior off.

.github/workflows/ci.yml

@@ -119,6 +119,7 @@ jobs:
   Windows:
     runs-on: windows-2022
     strategy:
+      fail-fast: false
       matrix:
         be: [mingw-gcc, msvc]
     steps: