Fix nanos6 cross-compilation for riscv

Even if we do an override to papi get the proper configure flags for
cross-compiling, the memory fences are not defined for risc-v:

mb.h:67:2: error: #error Need to define rmb for this architecture!

flake.nix

@@ -17,11 +17,7 @@
       legacyPackages.${system} = pkgs;
 
       hydraJobs = {
-        inherit (self.legacyPackages.${system}.bsc-ci) test pkgs;
+        inherit (self.legacyPackages.${system}.bsc-ci) test pkgs cross;
-
-        cross = self.lib.genAttrs [ "riscv64" ] (target:
-          self.legacyPackages.${system}.pkgsCross.${target}.bsc-ci.pkgs
-        );
       };
 
       # propagate nixpkgs lib, so we can do bscpkgs.lib

overlay.nix

@@ -96,6 +96,10 @@ in bscPkgs // {
     tests = final.runCommand "ci-tests" { }
       "printf '%s\n' ${toString (collect isDerivation final.bsc-ci.test)} > $out";
 
+    cross = prev.lib.genAttrs [ "riscv64" ] (target:
+      final.pkgsCross.${target}.bsc-ci.pkgs
+    );
+
     all = final.runCommand "ci-all" { }
     ''
       deps="${toString [ final.bsc-ci.pkgsList final.bsc-ci.tests ]}"