gitlab-ci: use haskell:8.8.4

1 job for master in 1 minute and 19 seconds (queued for 4 seconds)
Status Name Job ID Coverage
  Test
failed build #1083

00:01:19

 
Name Stage Failure
failed
build Test There is an unknown failure, please try again
$ mkdir -p /cache/root-stack
$ ln -s /cache/root-stack /root/.stack
$ stack test
Cloning f3a0c09be9a5ddd8ac76747915ea627d70ed56e6 from https://github.com/brendanhay/gogol.git
Cloning f3a0c09be9a5ddd8ac76747915ea627d70ed56e6 from https://github.com/brendanhay/gogol.git
Cloning f3a0c09be9a5ddd8ac76747915ea627d70ed56e6 from https://github.com/brendanhay/gogol.git
No compiler found, expected minor version match with ghc-8.6.5 (x86_64-tinfo6) (based on resolver setting in /builds/league/bookme/stack.yaml).
To install the correct GHC into /root/.stack/programs/x86_64-linux/, try running "stack setup" or use the "--install-ghc" flag. To use your system GHC installation, run "stack config set system-ghc --global true", or use the "--system-ghc" flag.
ERROR: Job failed: exit code 1