Support --semaphore #778
Description
The --jobs flag currently supports specifying different yet fixed numbers of jobs for Cabal's --jobs and GHC's -j flags.
As described in GHC Proposal 540, no fixed values for these flags result in optimal use of resources. Instead, the Cabal --semaphore flag allows GHC and Cabal to coordinate and share all available parallelism.
It would be nice if haskell-ci would generate CI configurations that use this flag. Conveniently, Cabal simply ignores the flag if GHC doesn't support it, so it can safely be passed to cabal configure unconditionally, as long as Cabal is at least as recent as 3.10.