Use pre-installed MinGW on Appveyor
Use pre-installed MinGW on Appveyor

Currently out Appveyor CI downloads and stashes a custom MinGW installation.
However the builder already provides both 64 and 32 bit installations of MinGW.
This patch changes our CI to use those instead.

I'm hoping this will fix issues where the g++ is broken due to the Appveyor
package caching semantics.

nickhutchinson/benchmark,disconnect3d/benchmark,ntx-/benchmark,efcs/benchmark,efcs/benchmark,disconnect3d/benchmark,google/benchmark,fuchsia-mirror/third_party-benchmark,ntx-/benchmark,google/benchmark,fuchsia-mirror/third_party-benchmark,ntx-/benchmark,nickhutchinson/benchmark,everbase/benchmark,nickhutchinson/benchmark-cxx03,everbase/benchmark,google/benchmark,nickhutchinson/benchmark,everbase/benchmark,nickhutchinson/benchmark,efcs/benchmark,nickhutchinson/benchmark-cxx03,nickhutchinson/benchmark-cxx03,mockingbirdnest/benchmark,codygray/benchmark,codygray/benchmark,ntx-/benchmark,efcs/benchmark,fuchsia-mirror/third_party-benchmark,mockingbirdnest/benchmark,fuchsia-mirror/third_party-benchmark,nickhutchinson/benchmark-cxx03,mockingbirdnest/benchmark,mockingbirdnest/benchmark,everbase/benchmark,disconnect3d/benchmark,disconnect3d/benchmark,codygray/benchmark,codygray/benchmark