Restore the GHC HEAD builder

[quasicomputational]

The GHC HEAD package in HVR's PPA has gone quite stale, so I didn't bother moving it over during the migration to Github Actions. haskell-CI/haskell-ci#458 is a request to freshen the package; if that happens, re-adding a HEAD builder would be quite nice to get early warning of breakage.

[andreasabel]

See also haskell-CI/haskell-ci#460 for a request to resurrect the head job.

[treeowl]

This would be especially helpful for packages like containers, which are distributed with GHC and not allowed to break (or warn) on the bleeding edge.