Specify path to stack.

See #110. [skip ci]

.appveyor.yml

@@ -12,7 +12,7 @@ install:
     - git clone --recursive git://git.haskell.org/ghc.git C:\msys64\home\ghc\tmp
     - bash -lc "mv /home/ghc/tmp/* /home/ghc"
     - cd C:\msys64\home\ghc
-    - stack exec -- mk/get-win32-tarballs.sh download x86_64
+    - C:\stack exec -- mk/get-win32-tarballs.sh download x86_64
 
 build_script:
     - bash -lc "cd /home/ghc && ./boot"