Update musl.nix (enableSystemd -> systemdSupport) (#2321)

This was changed in nixpkgs. We might be able to just outright ignore this, as it tires to detect systemd based on available libraries. This is just more explicit and tries to shield us against musl/systemd

Author: angerman

overlays/musl.nix

@@ -37,7 +37,7 @@ final: prev: prev.lib.optionalAttrs prev.stdenv.hostPlatform.isMusl ({
       NIX_LDFLAGS = "--push-state --as-needed -lstdc++ --pop-state";
       # without this collate.icu.utf8, and foreign_data will fail.
       LC_CTYPE = "C";
-    })).override { enableSystemd = false; gssSupport = false; };
+    })).override { systemdSupport = false; gssSupport = false; };
 
   openssl = prev.openssl.override { static = true; };