Switch back to hackage.haskell.org

's3.amazonaws.com' is not meant to serve clients around the world, as it
is hosted in the US only. This causes bandwidth to be severely throttled
for, amongst others, European users. This has been observed by users in
the following issue:

  commercialhaskell/stack#2240

At some periods, bandwidth drops to effectively zero, rendering Stack
unusable. Measurements can be found here:

  commercialhaskell/stack#2240 (comment)

'hackage.haskell.org' is served from Fastly's CDN, therefore mitigating
these issues.

src/Pantry.hs

@@ -295,7 +295,7 @@ defaultHackageSecurityConfig = HackageSecurityConfig
       , "fe331502606802feac15e514d9b9ea83fee8b6ffef71335479a2e68d84adc6b0"
       ]
   , hscKeyThreshold = 3
-  , hscDownloadPrefix = "https://s3.amazonaws.com/hackage.fpcomplete.com/"
+  , hscDownloadPrefix = "https://hackage.haskell.org/"
   , hscIgnoreExpiry = False
   }