Gitlab CI: work around cassava problem, use system-ghc

2 jobs for master in 10 seconds
Status Job ID Name Coverage
  Test
failed #582
build

00:00:10

failed #581
build

00:00:11

 
Name Stage Failure
failed
build Test There is an unknown failure, please try again
  method               = "GET"
proxy = Nothing
rawBody = False
redirectCount = 10
responseTimeout = ResponseTimeoutDefault
requestVersion = HTTP/1.1
}
(ConnectionFailure Network.BSD.getProtocolByName: does not exist (no such protocol name: tcp))
ERROR: Job failed: exit code 1