try to fix CI by not using nightly toolchains on windows

Author: Byron

.github/workflows/cross-platform-testing.yml

@@ -36,15 +36,15 @@ jobs:
             target: x86_64-apple-darwin
           - build: win-msvc
             os: windows-2019
-            rust: nightly
+            rust: stable
             target: x86_64-pc-windows-msvc
           - build: win-gnu
             os: windows-2019
-            rust: nightly-x86_64-gnu
+            rust: stable
             target: x86_64-pc-windows-gnu
           - build: win32-msvc
             os: windows-2019
-            rust: nightly
+            rust: stable
             target: i686-pc-windows-msvc
 
     steps: