Add workflow_dispatch option to mingw_static runner.

Author: cboulay

.github/workflows/mingw_static.yml

@@ -9,6 +9,7 @@ on:
       - '!.github/**'
       - '.github/workflows/mingw_static.yml'
   pull_request:
+  workflow_dispatch:
 
 concurrency:
   group: ${{ github.workflow }}-${{ github.head_ref || github.run_id }}
@@ -17,7 +18,7 @@ concurrency:
 jobs:
   build:
     name: MinGW batteries-included
-    runs-on: windows-2019
+    runs-on: windows-latest
 
     defaults:
       run: