Appveyor: Use the already-installed MinGW and Cygwin.

I don't know why we've been installing our own. According to this these are already installed: https://www.appveyor.com/docs/build-environment/#mingw-msys-cygwin

Also use super-test.sh to test Cygwin; get rid of appveyor-cygwin.sh which tests the wrong thing.

appveyor-cygwin.sh

-#! /usr/bin/env bash
-
-cmake -Hc++ -B./build-cygwin -G "Unix Makefiles" -DCMAKE_BUILD_TYPE=debug -DCMAKE_INSTALL_PREFIX=./capnproto-c++-cygwin
-cmake --build ./build-cygwin --config debug --target install -- -j2

appveyor.yml

@@ -3,12 +3,11 @@
 # See https://www.appveyor.com/docs/appveyor-yml/ for configuration options.
 #
 # This script configures AppVeyor to:
-#   - Download and unzip MinGW-w64 4.8.5 for x86_64, the minimum gcc version Cap'n Proto advertises
-#     support for.
 #   - Use CMake to ...
-#       build Cap'n Proto with MinGW.
 #       build Cap'n Proto with VS2017.
 #       build Cap'n Proto samples with VS2017.
+#       build Cap'n Proto with MinGW.
+#       build Cap'n Proto with Cygwin.
 
 version: "{build}"
 
@@ -24,13 +23,8 @@ image: Visual Studio 2017
 shallow_clone: true
 # Fetch repository as zip archive.
 
-cache:
-  - x86_64-4.8.5-release-win32-seh-rt_v4-rev0.7z
-
 environment:
-  MINGW_DIR: mingw64
-  MINGW_URL: https://sourceforge.net/projects/mingw-w64/files/Toolchains%20targetting%20Win64/Personal%20Builds/mingw-builds/4.8.5/threads-win32/seh/x86_64-4.8.5-release-win32-seh-rt_v4-rev0.7z/download
-  MINGW_ARCHIVE: x86_64-4.8.5-release-win32-seh-rt_v4-rev0.7z
+  MINGW_DIR: C:\mingw-w64\x86_64-7.2.0-posix-seh-rt_v5-rev1\mingw64
   BUILD_TYPE: debug
 
   matrix:
@@ -49,15 +43,11 @@ environment:
     - BUILD_NAME: cygwin
 
 install:
-  - if not exist "%MINGW_ARCHIVE%" appveyor DownloadFile "%MINGW_URL%" -FileName "%MINGW_ARCHIVE%"
-  - 7z x -y "%MINGW_ARCHIVE%" > nul
   - ps: Get-Command sh.exe -All | Remove-Item
   # CMake refuses to generate MinGW Makefiles if sh.exe is in the PATH
-  - if "%BUILD_NAME%"=="cygwin" appveyor DownloadFile "http://cygwin.com/setup-x86_64.exe" -FileName "C:\cygwin64\setup-x86_64.exe"
-  - if "%BUILD_NAME%"=="cygwin" C:\cygwin64\setup-x86_64.exe --quiet-mode --no-shortcuts --upgrade-also --root "C:\cygwin64" --packages cmake
 
 before_build:
-  - set PATH=%CD%\%MINGW_DIR%\bin;%PATH%
+  - set PATH=%MINGW_DIR%\bin;%PATH%
   - set BUILD_DIR=build-%BUILD_NAME%
   - set INSTALL_PREFIX=%CD%\capnproto-c++-%BUILD_NAME%
   - cmake --version
@@ -74,8 +64,8 @@ build_script:
   - if NOT "%BUILD_NAME%"=="cygwin" cmake -Hc++/samples -B%BUILD_DIR%-samples -G "%CMAKE_GENERATOR%" -DCMAKE_BUILD_TYPE=%BUILD_TYPE% -DCMAKE_PREFIX_PATH=%INSTALL_PREFIX%
   - if NOT "%BUILD_NAME%"=="cygwin" cmake --build %BUILD_DIR%-samples --config %BUILD_TYPE%
 
-  # Cygwin build
-  - if "%BUILD_NAME%"=="cygwin" C:\cygwin64\bin\bash -lc 'cd /cygdrive/c/projects/capnproto; ./appveyor-cygwin.sh'
+  # Cygwin build -- use super-test.sh like other Unix builds.
+  - if "%BUILD_NAME%"=="cygwin" C:\cygwin64\bin\bash -lc 'cd /cygdrive/c/projects/capnproto; ./super-test.sh -j2 quick'
 
 test_script:
   - timeout /t 2