ci: Speed up builds using ghc-options: -j

Building xmonad-contrib benefits greatly from compiling modules in
parallel. Vanessa McHale suggested setting ghc-options in
xmonad-contrib.cabal in #431, but that's bad practice—these options
should be set by the user/environment where it's being built, not
selfishly hardcoded for specific package. In stack.yaml and
cabal.haskell-ci, we know for sure that xmonad-contrib is the terminal
(leaf) library, so we can set it there and speed up builds in our CI and
also for anyone who uses our stack.yaml (not that anyone should).

Closes: https://github.com/xmonad/xmonad-contrib/pull/431

.github/workflows/haskell-ci.yml

@@ -142,7 +142,8 @@ jobs:
             branch:   master
 
           package xmonad-contrib
-            flags: +pedantic
+            flags:       +pedantic
+            ghc-options: -j
           EOF
           $HCPKG list --simple-output --names-only | perl -ne 'for (split /\s+/) { print "constraints: $_ installed\n" unless /^(xmonad-contrib)$/; }' >> cabal.project.local
           cat cabal.project

cabal.haskell-ci

@@ -9,3 +9,4 @@ apt:
 raw-project
   package xmonad-contrib
     flags: +pedantic
+    ghc-options: -j

stack-master.yaml

@@ -9,3 +9,6 @@ packages:
 extra-deps:
 - github: xmonad/xmonad
   commit: master@{today}
+
+ghc-options:
+  xmonad-contrib: '-j'

stack.yaml

@@ -15,3 +15,6 @@ nix:
     - xorg.libXrandr
     - xorg.libXrender
     - xorg.libXScrnSaver
+
+ghc-options:
+  xmonad-contrib: '-j'