Create ghc-tarballs/perl folder.

See #110. [skip ci]

.appveyor.yml

@@ -15,6 +15,7 @@ install:
     - cd C:\msys64\home\ghc
     - stack exec -- perl boot
     - bash -lc "cd /home/ghc; mkdir -p ghc-tarballs/mingw-w64/x86_64/"
+    - bash -lc "cd /home/ghc; mkdir -p ghc-tarballs/perl/"
     - bash -lc "cd /home/ghc; exec 0</dev/null; ./configure --enable-tarballs-autodownload"
     - ghc   --version
     - stack --version