flake.nix: remove power64 from from nixos check due to broken package

(cherry picked from commit 4bf6540)

flake.nix

@@ -46,7 +46,8 @@
 
       checks = forAllSystems (system: {
         tarball = jobs.${system}.tarball;
-      } // lib.optionalAttrs (self.legacyPackages.${system}.stdenv.isLinux) {
+        # Exclude power64 due to "libressl is not available on the requested hostPlatform" with hostPlatform being power64
+      } // lib.optionalAttrs (self.legacyPackages.${system}.stdenv.isLinux && !self.legacyPackages.${system}.targetPlatform.isPower64) {
         # Test that ensures that the nixosSystem function can accept a lib argument
         # Note: prefer not to extend or modify `lib`, especially if you want to share reusable modules
         #       alternatives include: `import` a file, or put a custom library in an option or in `_module.args.<libname>`