Try to fix AppVeyor.

See https://github.com/haskell/cabal/commit/123291c0939789aa7c3e4970c4a22adf50c0a9ee#comments.

(cherry picked from commit 8739c9b6)

appveyor.yml

 install:
   # Using '-y' and 'refreshenv' as a workaround to:
   # https://github.com/haskell/cabal/issues/3687
-  - choco install -y cabal
-  - choco install -y ghc --version 8.0.2
+  - choco source add -n mistuke -s https://www.myget.org/F/mistuke/api/v2
+  - choco install -y ghc --version 8.0.2 --ignore-dependencies
+  - choco install -y cabal-head -pre
   - refreshenv
   # See http://help.appveyor.com/discussions/problems/6312-curl-command-not-found#comment_42195491
   # NB: Do this after refreshenv, otherwise it will be clobbered!

cabal-testsuite/cabal-testsuite.cabal

@@ -64,7 +64,7 @@ executable cabal-tests
   build-depends:
     async,
     base,
-    Cabal >= 2.3,
+    Cabal == 2.5.0.0,
     clock,
     filepath,
     process,
@@ -75,5 +75,5 @@ executable cabal-tests
   default-language: Haskell2010
 
 custom-setup
-  setup-depends: Cabal >= 1.25,
+  setup-depends: Cabal == 2.5.0.0,
                  base