Use mingw64_shell.bat for running scripts.

See #110. [skip ci]

.appveyor.yml

@@ -3,7 +3,7 @@ clone_folder: "C:\\msys64\\home\\ghc\\shake-build"
 install:
     - cd
     - set MSYSTEM=MINGW64
-    - set PATH=C:\msys64\mingw64\bin;C:\msys64\usr\local\bin;C:\msys64\usr\bin;%PATH%
+    - set PATH=C:\msys64\mingw64\bin;C:\msys64\usr\bin;C:\msys64;%PATH%
     - set PATH=%PATH%;C:\Users\appveyor\AppData\Roaming\local\bin\;%APPVEYOR_BUILD_FOLDER%
     - curl -ostack.zip -LsS --insecure https://www.stackage.org/stack/windows-x86_64
     - 7z x stack.zip stack.exe
@@ -14,7 +14,8 @@ install:
     - bash -lc "mv /home/ghc/tmp/* /home/ghc"
     - cd C:\msys64\home\ghc
     - stack exec -- perl boot
-    - bash -lc "cd /home/ghc; exec 0</dev/null; ./configure --enable-tarballs-autodownload"
+    - mingw64_shell.bat "cd /home/ghc; exec 0</dev/null; ./configure --enable-tarballs-autodownload"
+    # - bash -lc "cd /home/ghc; exec 0</dev/null; ./configure --enable-tarballs-autodownload"
     - ghc   --version
     - stack --version
     - alex  --version