This was temporarily disabled in 229d52ff07 and 086db3123b because we
couldn't keep xmonad-contrib backwards compatible with xmonad 0.15 any
more, but xmonad 0.17.0 is out now and we can try to stay compatible for
a while.
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
Turns out `commit: master` doesn't really mean latest master, because
it's cached. It sometimes works, because when the GHA cache comes from
a job that depended on xmonad-0.15, then the master archive isn't cached
and is fetched. When the cache comes from a job that previously fetched
master, it's not fetched again.
The least ugly way to fix it I found is to use the `[<refname>]@{<date>}`
git revision format, and substitute current date/time before invoking
stack. To make stack-master.yaml valid without the substitution, we use
`master@{today}` which is a valid git revision that GitHub understands.
