workflows: Downgrade mingw to workaround Windows failure

Because of golang/go#46099 'make check' on
Windows in GitHub Actions is failing with '==34372==ERROR:
ThreadSanitizer failed to allocate 0x000003061000 (50728960) bytes at
0x200dabe7d0000 (error code: 87)'

This commit downgrades mingw to 10.0.20 in order to avoid this problem.
This will need to be reverted once this is fixed in mingw or golang.

.github/workflows/make-check-win.yml

@@ -17,6 +17,8 @@ jobs:
         go:
           - 1.17
     steps:
+      - name: Downgrade mingw to workaround https://github.com/golang/go/issues/46099
+        run: choco install mingw --version 10.2.0 --allow-downgrade
       - name: Check out repository code
         uses: actions/checkout@v2
       - name: Set up Go