ci: change cache key for windows/mingw

ci: change cache key for windows/mingw

Should fix the following current failures:
- 2020-05-04T22:32:02.9490248Z C:\Program Files (x86)\Microsoft Visual Studio\2019\Enterprise\MSBuild\Microsoft\VC\v160\Microsoft.CppCommon.targets(231,5): error MSB6006: "cmd.exe" exited with code 3. [D:\a\libevent\libevent\build\regress.vcxproj]
- cmake : /usr/bin/sh: /C/hostedtoolcache/windows/Python/3.7.6/x64/python.exe: No such file or directory

P.S. I guess python has another path, but on my fork it is the same and
it passes.