Run CI builds with Stackage Nightly.

Unfortunately, our CI builds have a hard-coded assumption that all kinds of
LTS-X builds can be performed with the same stack.yaml file, which is not true.
We need to use specialized build instructions for each version of the
environment we're building in, e.g. stack-lts-16.yaml, stack-nightly.yaml, and
so on. Making that change is not super hard, but it's not completely trivial
either because the hooks use a hash of the stack file to determine whether the
cash is still valid, etc., and I'm not entirely sure about the best way to
adapt that code to support multiple YAML files.

For the time being, the most important build IMHO is Stackage Nightly,
because (a) we really want to support the nightly snapshot since (b) it can
compile the genmanpage executable relatively easily.

.github/workflows/tests.yml

@@ -14,7 +14,7 @@ jobs:
       fail-fast: false
       matrix:
         os: [ubuntu-latest]
-        resolver: [lts-16, lts-15, lts-14, lts-12]
+        resolver: [nightly]
 
     steps:
       - name: Clone project