maintainers/scripts/test-configurations.nix: skip binary GHCs

While being able to test them is neat (on x86_64-linux they work very
well, actually), we usually don't want to do this, since the set is
only (recommended to be) used to bootstrap GHC. Consequently there is
almost no binary cache and testing them mostly leads to unenlightening
and seemingly endless compilation.

maintainers/scripts/haskell/test-configurations.nix

@@ -39,6 +39,12 @@
      nix-build maintainers/scripts/haskell/test-configurations.nix \
        --arg config '{ allowBroken = false; }' --keep-going
 
+   By default the haskell.packages.ghc*Binary sets used for bootstrapping GHC
+   are _not_ tested. You can change this using:
+
+     nix-build maintainers/scripts/haskell/test-configurations.nix \
+       --arg skipBinaryGHCs false --keep-going
+
 */
 { files ? [
     "configuration-common.nix"
@@ -48,6 +54,7 @@
 , nixpkgsPath ? ../../..
 , config ? { allowBroken = true; }
 , skipEvalErrors ? true
+, skipBinaryGHCs ? true
 }:
 
 let
@@ -70,8 +77,11 @@ let
         builtins.match "ghc-([0-9]+).([0-9]+).x" configName
       );
       # return all package sets under haskell.packages matching the version components
-      setsForVersion = builtins.map (name: pkgs.haskell.packages.${name}) (
-        builtins.filter (lib.hasPrefix "ghc${configVersion}") (
+      setsForVersion =  builtins.map (name: pkgs.haskell.packages.${name}) (
+        builtins.filter (setName:
+          lib.hasPrefix "ghc${configVersion}" setName
+          && (skipBinaryGHCs -> !(lib.hasInfix "Binary" setName))
+        ) (
           builtins.attrNames pkgs.haskell.packages
         )
       );