release-haskell.nix: don't try to build pkgsMusl on non x86_64-linux

Since 322bab8284, ghc no longer self
reports its restricted platforms for e.g. pkgsMusl, so we need to
reflect this in the jobset in order to keep the evaluation errors tab
clean.

pkgs/top-level/release-haskell.nix

@@ -394,19 +394,7 @@ let
       };
 
       # GHCs linked to musl.
-      pkgsMusl.haskell.compiler = lib.recursiveUpdate
-        (packagePlatforms pkgs.pkgsMusl.haskell.compiler)
-        {
-          # remove musl ghc865Binary since it is known to be broken and
-          # causes an evaluation error on darwin.
-          ghc865Binary = {};
-
-          ghcjs = {};
-          ghcjs810 = {};
-        };
-
-      # Get some cache going for MUSL-enabled GHC.
-      pkgsMusl.haskellPackages =
+      pkgsMusl =
         removePlatforms
           [
             # pkgsMusl is compiled natively with musl.  It is not
@@ -420,11 +408,26 @@ let
             "aarch64-darwin"
           ]
           {
-            inherit (packagePlatforms pkgs.pkgsMusl.haskellPackages)
-              hello
-              lens
-              random
-              ;
+            haskell.compiler = lib.recursiveUpdate
+              (packagePlatforms pkgs.pkgsMusl.haskell.compiler)
+              {
+                # remove musl ghc865Binary since it is known to be broken and
+                # causes an evaluation error on darwin.
+                ghc865Binary = {};
+
+                ghcjs = {};
+                ghcjs810 = {};
+              };
+
+            # Get some cache going for MUSL-enabled GHC.
+            haskellPackages =
+              {
+                inherit (packagePlatforms pkgs.pkgsMusl.haskellPackages)
+                  hello
+                  lens
+                  random
+                ;
+              };
           };
 
       # Test some statically linked packages to catch regressions