ci: fix mingw install path
looks like this changed when mingw was bumped to 8.1.0
$ErrorActionPreference = "Stop"
trap { $host.SetShouldExit(1) }
-$env:Path += ";C:\Go\bin;C:\Program Files\Git\cmd;C:\tools\mingw64\bin"
+$env:Path += ";C:\Go\bin;C:\Program Files\Git\cmd;C:\ProgramData\chocolatey\lib\mingw\tools\install\mingw64\bin"
$env:GOPATH = "$pwd\gopath"
$env:Path += ";$pwd\gopath\bin"
