Change to use windows-2019 to fix CI. (#252)

This commit is an attempt to work around flakiness in the Windows CI relating
to downloading Wasmtime release assets.

See actions/runner-images#7007.

Author: peterhuene

.github/workflows/main.yml

@@ -32,10 +32,10 @@ jobs:
             os: macos-latest
             config: release
           - build: windows-debug
-            os: windows-latest
+            os: windows-2019
             config: debug
           - build: windows-release
-            os: windows-latest
+            os: windows-2019
             config: release
     steps:
     - uses: actions/checkout@v2