enable all-features in nix for CI builds

CI is running `cargo build --all-features`, so we should be passing all
the features to nix as well.

The only thing this currently affects is the jemalloc_prof feature, but if
we add any non-default features that affect nix in the future they should
also be handled correctly now.

flake.nix

@@ -188,5 +188,9 @@
         );
 
       devShells.default = mkDevShell scopeHostStatic;
+      devShells.all-features = mkDevShell
+        (scopeHostStatic.overrideScope (final: prev: {
+          main = prev.main.override { all_features = true; };
+        }));
     });
 }