CI: downgrade to Visual Studio 2019

Works around build errors in libobjc2 and libdispatch under Visual Studio 17.1 due to stdatomic.h being unsupported in C:
https://github.com/gnustep/libobjc2/issues/225

.github/workflows/main.yml

@@ -35,7 +35,7 @@ jobs:
 
           - name: Windows MinGW GCC i686
             allow-test-failures: true
-            os: windows-latest
+            os: windows-2019
             arch: i686
             msystem: MINGW32
             library-combo: gnu-gnu-gnu
@@ -44,7 +44,7 @@ jobs:
 
           - name: Windows MinGW GCC x86_64
             allow-test-failures: true
-            os: windows-latest
+            os: windows-2019
             arch: x86_64
             msystem: MINGW64
             library-combo: gnu-gnu-gnu
@@ -53,7 +53,7 @@ jobs:
 
           - name: Windows MSVC Clang gnustep-2.0 i686
             allow-test-failures: true
-            os: windows-latest
+            os: windows-2019
             arch: x86
             host: i686-pc-windows
             library-combo: ng-gnu-gnu
@@ -65,7 +65,7 @@ jobs:
 
           - name: Windows MSVC Clang gnustep-2.0 x86_64
             allow-test-failures: true
-            os: windows-latest
+            os: windows-2019
             arch: x64
             host: x86_64-pc-windows
             library-combo: ng-gnu-gnu