Add 9.8 to CI, remove 9.2 and 9.4 (#569)

* Add 9.8 to CI, remove 9.2 and 9.4

We're moving towards 9.8, we're not going to use 9.2 or 9.4 in the long
run.

Since 9.8 is only going to work for a small set of packages to begin
with, I added an `experimental-compilers` list. Things in that list are
off by default but you can turn them on in `exceptions`.

`cardano-prelude` builds with 9.8 now, so that's our first test!

* Put 9.2 back

flake.nix

@@ -3,7 +3,7 @@
 
   inputs = {
     nixpkgs.follows = "haskell-nix/nixpkgs";
-    flake-utils.follows = "haskell-nix/flake-utils";
+    flake-utils = { url = "github:numtide/flake-utils"; };
 
     foliage = {
       url = "github:input-output-hk/foliage";
@@ -61,7 +61,9 @@
 
       # type CompilerName = String
       # compilers :: [CompilerName]
-      compilers = [ "ghc810" "ghc92" "ghc96" ];
+      compilers = [ "ghc810" "ghc92" "ghc96" "ghc98" ];
+      # compilers which we don't build for by default
+      experimental-compilers = [ "ghc98" ];
 
       # Add exceptions to the CI here.
       #
@@ -72,6 +74,9 @@
       #   compilers (defined above) are included.
       #
       exceptions = {
+        cardano-prelude = {
+          ghc98.enabled = true;
+        };
         plutus-ledger = {
           ghc92.enabled = false;
           ghc96.enabled = false;
@@ -160,7 +165,10 @@
                 (name: _v:
                   lib.attrByPath
                     [ name compiler "enabled" ]
-                    true
+                    # the default setting depends on whether or not the compiler is
+                    # experimental. Experimental compilers are disabled by default,
+                    # non-experimental compilers are enabled by default
+                    (!(builtins.elem compiler experimental-compilers))
                     exceptions)
                 pkg-versions;
           in