tmuxPlugins: allow simple overrideAttrs without function

Regular `overrideAttrs` also supports passing a plain attribute set.

Before this PR:

```
nix-repl> tmuxPlugins.battery.overrideAttrs { pname = "my-battery"; }
error: attempt to call something which is not a function but a set: {
pname = "my-battery"; }
```

After, it works.

pkgs/misc/tmux-plugins/default.nix

@@ -16,7 +16,7 @@ let
       rtp = "${derivation}/${path}/${rtpFilePath}";
     }
     // {
-      overrideAttrs = f: mkTmuxPlugin (attrs // f attrs);
+      overrideAttrs = f: mkTmuxPlugin (attrs // (if lib.isFunction f then f attrs else f));
     };
 
   mkTmuxPlugin =