forgot to set up Mingw-w64

.appveyor.yml

@@ -19,6 +19,10 @@ clone_depth: 10
 install:
   - git submodule update --init --recursive
 
+  # use the newest versions documented here: https://www.appveyor.com/docs/windows-images-software/#mingw-msys-cygwin
+  - IF "%PLATFORM%" == "x86" SET PATH=C:\mingw-w64\i686-6.3.0-posix-dwarf-rt_v5-rev1\mingw32\bin;%PATH%
+  - IF "%PLATFORM%" == "x64" SET PATH=C:\mingw-w64\x86_64-8.1.0-posix-seh-rt_v6-rev0\mingw64\bin;%PATH%
+
   # build nim from our own branch - this to avoid the day-to-day churn and
   # regressions of the fast-paced Nim development while maintaining the
   # flexibility to apply patches
@@ -29,6 +33,7 @@ install:
 build_script:
   - cd C:\projects\%APPVEYOR_PROJECT_SLUG%
   - nimble install -y
+
 test_script:
   - nimble test